Remove unused hash<std::string>.
Remove unused hash<std::string>.

ExternalStringHashMap used to store std::strings long ago.  Since it
doesn't anymore, this specialization isn't needed.  No behavior change.

ninja-build/ninja,fuchsia-mirror/third_party-ninja,mohamed/ninja,Qix-/ninja,mgaunard/ninja,nafest/ninja,iwadon/ninja,mydongistiny/ninja,juntalis/ninja,nafest/ninja,maruel/ninja,liukd/ninja,sxlin/dist_ninja,autopulated/ninja,dendy/ninja,bradking/ninja,ndsol/subninja,ninja-build/ninja,atetubou/ninja,automeka/ninja,iwadon/ninja,AoD314/ninja,sgraham/ninja,mohamed/ninja,lizh06/ninja,kissthink/ninja,ndsol/subninja,juntalis/ninja,hnney/ninja,jimon/ninja,hnney/ninja,ignatenkobrain/ninja,ninja-build/ninja,sxlin/dist_ninja,ignatenkobrain/ninja,kissthink/ninja,mydongistiny/ninja,metti/ninja,tfarina/ninja,ctiller/ninja,liukd/ninja,ilor/ninja,lizh06/ninja,bmeurer/ninja,bmeurer/ninja,hnney/ninja,autopulated/ninja,ThiagoGarciaAlves/ninja,tfarina/ninja,jimon/ninja,dorgonman/ninja,synaptek/ninja,dendy/ninja,vvvrrooomm/ninja,bradking/ninja,bradking/ninja,mohamed/ninja,ignatenkobrain/ninja,fuchsia-mirror/third_party-ninja,lizh06/ninja,Ju2ender/ninja,ctiller/ninja,jendrikillner/ninja,lizh06/ninja,atetubou/ninja,martine/ninja,martine/ninja,moroten/ninja,ilor/ninja,jendrikillner/ninja,sxlin/dist_ninja,automeka/ninja,ctiller/ninja,kissthink/ninja,mydongistiny/ninja,moroten/ninja,mydongistiny/ninja,dorgonman/ninja,ignatenkobrain/ninja,jendrikillner/ninja,AoD314/ninja,maruel/ninja,nicolasdespres/ninja,ThiagoGarciaAlves/ninja,nicolasdespres/ninja,hnney/ninja,mgaunard/ninja,liukd/ninja,dendy/ninja,nafest/ninja,ilor/ninja,Qix-/ninja,tfarina/ninja,dendy/ninja,martine/ninja,moroten/ninja,Maratyszcza/ninja-pypi,automeka/ninja,sxlin/dist_ninja,mgaunard/ninja,dorgonman/ninja,iwadon/ninja,sgraham/ninja,ilor/ninja,iwadon/ninja,maruel/ninja,fuchsia-mirror/third_party-ninja,nafest/ninja,maruel/ninja,jimon/ninja,Ju2ender/ninja,ThiagoGarciaAlves/ninja,nico/ninja,juntalis/ninja,colincross/ninja,metti/ninja,synaptek/ninja,liukd/ninja,fuchsia-mirror/third_party-ninja,sxlin/dist_ninja,ctiller/ninja,bmeurer/ninja,sgraham/ninja,automeka/ninja,sxlin/dist_ninja,juntalis/ninja,Ju2ender/ninja,ThiagoGarciaAlves/ninja,colincross/ninja,martine/ninja,AoD314/ninja,Ju2ender/ninja,mohamed/ninja,bradking/ninja,vvvrrooomm/ninja,bmeurer/ninja,sgraham/ninja,colincross/ninja,nico/ninja,Maratyszcza/ninja-pypi,synaptek/ninja,ninja-build/ninja,vvvrrooomm/ninja,nico/ninja,atetubou/ninja,jendrikillner/ninja,kissthink/ninja,nicolasdespres/ninja,autopulated/ninja,Maratyszcza/ninja-pypi,vvvrrooomm/ninja,dorgonman/ninja,colincross/ninja,Qix-/ninja,Qix-/ninja,Maratyszcza/ninja-pypi,ndsol/subninja,nico/ninja,ndsol/subninja,jimon/ninja,tfarina/ninja,metti/ninja,AoD314/ninja,atetubou/ninja,moroten/ninja,synaptek/ninja,sxlin/dist_ninja,metti/ninja,autopulated/ninja,mgaunard/ninja,nicolasdespres/ninja