core: delete deprecated pickSubchannel() (#2849)
core: delete deprecated pickSubchannel() (#2849)

The deprecation happens in 1.2.0. Now we can delete it for the next release.
zhangkun83/grpc-java,rmichela/grpc-java,zpencer/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,simonhorlick/grpc-java,ejona86/grpc-java,pieterjanpintens/grpc-java,grpc/grpc-java,ejona86/grpc-java,nmittler/grpc-java,elandau/grpc-java,rmichela/grpc-java,zhangkun83/grpc-java,zpencer/grpc-java,stanley-cheung/grpc-java,pieterjanpintens/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java,zpencer/grpc-java,dapengzhang0/grpc-java,nmittler/grpc-java,elandau/grpc-java,simonhorlick/grpc-java,simonhorlick/grpc-java,pieterjanpintens/grpc-java,grpc/grpc-java,rmichela/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,nmittler/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,grpc/grpc-java,elandau/grpc-java,simonhorlick/grpc-java,elandau/grpc-java,rmichela/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,pieterjanpintens/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,zpencer/grpc-java