remove horizon exercises
remove horizon exercises

The horizon team keeps changing how the UI is exposed. This exercise
keeps lagging and doesn't really test anything useful any more. Just
delete it.

Change-Id: Id62904868f1d4b39e33d2ad63340b5ee2177fb56

pczerkas/devstack,dirkmueller/devstack,upadhyay-prashant/devstack,richliu/devstack,olivierlemasle/devstack,nawawi/openstack,prithivm/branch01,group-policy/devstack,avvocatodemarchis/devstack,vsham20/devstack,varunarya10/devstack,j4/devstack,costingalan/devstack,nuxeh/devstack,olivierlemasle/devstack,dirkmueller/devstack,kplus/devstack-moonshot,bigswitch/devstack,TwinkleChawla/devstack,mc2014/devstack,upadhyay-prashant/devstack,bswartz/devstack,openstack-dev/devstack,NaokiMiyata/devstack,bljgaurav/openstack-test,rushiagr/devstack,noironetworks/devstack,eharney/devstack,neerja28/Devstack_GlusterFS,liuquansheng47/devstack,flavio-fernandes/devstack,pacinete/devstack,TwinkleChawla/devstack,herrsechs/devstack,avvocatodemarchis/devstack,flavio-fernandes/devstack,atulpatil301/devstack,mssumanth/devstack,bq-xiao/devstack,parul24/devstack,bigswitch/devstack,HPCHub/devstack,vishnugonela/devstack,NaokiMiyata/devstack,thomasem/devstack,BSJAIN92/OpenStack,richliu/devstack,j4/devstack,rushiagr/devstack,sequenceiq/devstack,mssumanth/devstack,nawawi/openstack,mc2014/devstack,kplus/devstack-moonshot,andrewrothstein/devstack,wenhuizhang/devstack,dtroyer/devstack,wenhuizhang/devstack,BSJAIN92/OpenStack,mmasaki/devstack,LoHChina/devstack,varunarya10/devstack,group-policy/devstack,liuquansheng47/devstack,bljgaurav/openstack-test,bswartz/devstack,dtroyer/devstack,samgoon/devstack,noironetworks/devstack,parul24/devstack,atulpatil301/devstack,openstack-dev/devstack,herrsechs/devstack,pczerkas/devstack,CloudServer/devstack,ozamiatin/devstack,CloudServer/devstack,vishnugonela/devstack,nuxeh/devstack,vsham20/devstack,Mahito/devstack,pombredanne/devstack,costingalan/devstack,neerja28/Devstack_GlusterFS,samgoon/devstack,Mahito/devstack,sequenceiq/devstack,bq-xiao/devstack,LoHChina/devstack,mmasaki/devstack,ozamiatin/devstack,pacinete/devstack,andrewrothstein/devstack,thomasem/devstack,eharney/devstack,pombredanne/devstack,HPCHub/devstack,prithivm/branch01