Cleanup AmpStoryVariableService private field (#14810)
Cleanup AmpStoryVariableService private field (#14810)

Seems like a leftover from 97f6f61b89.
rsimha-amp/amphtml,smartadserver/amphtml,media-net/amphtml,BenDz/amphtml,ampproject/amphtml,widespace-os/amphtml,erwinmombay/amphtml,jridgewell/amphtml,alanorozco/amphtml,aghassemi/amphtml,honeybadgerdontcare/amphtml,luciancrasovan/amphtml,kamtschatka/amphtml,yieldbot/amphtml,danielrozenberg/amphtml,taboola/amphtml,dotandads/amphtml,tlong2/amphtml,luciancrasovan/amphtml,jmadler/amphtml,dotandads/amphtml,kamtschatka/amphtml,Gregable/amphtml,alanorozco/amphtml,Gregable/amphtml,adup-tech/amphtml,mhc-gh/amphtml,voyagegroup/amphtml,bpaduch/amphtml,techhtml/amphtml,cory-work/amphtml,chaveznvg/amphtml,zhouyx/amphtml,danielrozenberg/amphtml,chaveznvg/amphtml,jridgewell/amphtml,zhouyx/amphtml,bpaduch/amphtml,aghassemi/amphtml,jpettitt/amphtml,Gregable/amphtml,cory-work/amphtml,mhc-gh/amphtml,zhouyx/amphtml,yandex-pcode/amphtml,pedjoni8/amphtml,sharethrough/amphtml,mhc-gh/amphtml,voyagegroup/amphtml,prateekbh/amphtml,bpaduch/amphtml,dotandads/amphtml,koshka/amphtml,yandex-pcode/amphtml,cvializ/amphtml,media-net/amphtml,kamtschatka/amphtml,ampproject/amphtml,lannka/amphtml,danielrozenberg/amphtml,yieldbot/amphtml,erwinmombay/amphtml,alanorozco/amphtml,jpettitt/amphtml,mhc-gh/amphtml,dotandads/amphtml,techhtml/amphtml,techhtml/amphtml,luciancrasovan/amphtml,zhangsu/amphtml,luciancrasovan/amphtml,aghassemi/amphtml,yieldbot/amphtml,jpettitt/amphtml,lannka/amphtml,src-code/amphtml,jpettitt/amphtml,taboola/amphtml,chaveznvg/amphtml,prateekbh/amphtml,zhangsu/amphtml,yandex-pcode/amphtml,widespace-os/amphtml,sebastianbenz/amphtml,yandex-pcode/amphtml,laterpay/amphtml,Gregable/amphtml,jmadler/amphtml,voyagegroup/amphtml,mhc-gh/amphtml,zhouyx/amphtml,jridgewell/amphtml,prateekbh/amphtml,honeybadgerdontcare/amphtml,voyagegroup/amphtml,aghassemi/amphtml,yandex-pcode/amphtml,jridgewell/amphtml,lannka/amphtml,src-code/amphtml,lannka/amphtml,bpaduch/amphtml,jmadler/amphtml,tlong2/amphtml,nexxtv/amphtml,laterpay/amphtml,cory-work/amphtml,sharethrough/amphtml,erwinmombay/amphtml,sharethrough/amphtml,src-code/amphtml,honeybadgerdontcare/amphtml,adup-tech/amphtml,adup-tech/amphtml,tlong2/amphtml,aghassemi/amphtml,sebastianbenz/amphtml,pedjoni8/amphtml,pedjoni8/amphtml,sharethrough/amphtml,alanorozco/amphtml,prateekbh/amphtml,adup-tech/amphtml,adup-tech/amphtml,media-net/amphtml,cory-work/amphtml,aghassemi/amphtml,luciancrasovan/amphtml,media-net/amphtml,smartadserver/amphtml,yandex-pcode/amphtml,BenDz/amphtml,chaveznvg/amphtml,koshka/amphtml,src-code/amphtml,jpettitt/amphtml,voyagegroup/amphtml,koshka/amphtml,cory-work/amphtml,taboola/amphtml,mhc-gh/amphtml,Gregable/amphtml,Gregable/amphtml,BenDz/amphtml,nexxtv/amphtml,alanorozco/amphtml,BenDz/amphtml,chaveznvg/amphtml,Gregable/amphtml,cvializ/amphtml,zhangsu/amphtml,bpaduch/amphtml,media-net/amphtml,yieldbot/amphtml,jmadler/amphtml,koshka/amphtml,cory-work/amphtml,dotandads/amphtml,smartadserver/amphtml,techhtml/amphtml,sebastianbenz/amphtml,bpaduch/amphtml,techhtml/amphtml,honeybadgerdontcare/amphtml,jpettitt/amphtml,ampproject/amphtml,taboola/amphtml,rsimha-amp/amphtml,rsimha-amp/amphtml,lannka/amphtml,jridgewell/amphtml,sharethrough/amphtml,kamtschatka/amphtml,zhouyx/amphtml,ampproject/amphtml,pedjoni8/amphtml,media-net/amphtml,dotandads/amphtml,BenDz/amphtml,nexxtv/amphtml,voyagegroup/amphtml,jpettitt/amphtml,prateekbh/amphtml,taboola/amphtml,jridgewell/amphtml,sebastianbenz/amphtml,rsimha-amp/amphtml,techhtml/amphtml,mhc-gh/amphtml,laterpay/amphtml,cvializ/amphtml,laterpay/amphtml,honeybadgerdontcare/amphtml,rsimha-amp/amphtml,luciancrasovan/amphtml,nexxtv/amphtml,cvializ/amphtml,zhouyx/amphtml,sebastianbenz/amphtml,tlong2/amphtml,lannka/amphtml,laterpay/amphtml,smartadserver/amphtml,chaveznvg/amphtml,zhangsu/amphtml,laterpay/amphtml,smartadserver/amphtml,koshka/amphtml,taboola/amphtml,ampproject/amphtml,zhangsu/amphtml,alanorozco/amphtml,erwinmombay/amphtml,rsimha-amp/amphtml,nexxtv/amphtml,mhc-gh/amphtml,pedjoni8/amphtml,media-net/amphtml,koshka/amphtml,erwinmombay/amphtml,erwinmombay/amphtml,widespace-os/amphtml,media-net/amphtml,bpaduch/amphtml,cvializ/amphtml,jpettitt/amphtml,pedjoni8/amphtml,prateekbh/amphtml,widespace-os/amphtml,tlong2/amphtml,ampproject/amphtml,taboola/amphtml,danielrozenberg/amphtml,jmadler/amphtml,src-code/amphtml,sebastianbenz/amphtml,adup-tech/amphtml,luciancrasovan/amphtml,rsimha-amp/amphtml,nexxtv/amphtml,ampproject/amphtml,lannka/amphtml,widespace-os/amphtml,kamtschatka/amphtml,honeybadgerdontcare/amphtml,alanorozco/amphtml,kamtschatka/amphtml,danielrozenberg/amphtml,prateekbh/amphtml,src-code/amphtml,nexxtv/amphtml,jridgewell/amphtml,cory-work/amphtml,zhangsu/amphtml,voyagegroup/amphtml,tlong2/amphtml,ampproject/amphtml,honeybadgerdontcare/amphtml,yandex-pcode/amphtml,jmadler/amphtml,danielrozenberg/amphtml,danielrozenberg/amphtml,yieldbot/amphtml,zhouyx/amphtml,smartadserver/amphtml,BenDz/amphtml,yieldbot/amphtml,sharethrough/amphtml,Gregable/amphtml,alanorozco/amphtml,cvializ/amphtml,widespace-os/amphtml,danielrozenberg/amphtml