Delete the generated files only before generateProto.
Delete the generated files only before generateProto.

Previously generated files were deleted at configuration time. Running
non-build tasks such as ``clean`` or ``tasks`` would delete the
generated files and tracked by git, which is annoying. Now we delete
them only before the ``generateProto`` task, which solves the problem.

After this change, the case that ``generateProto`` is not executed at
all, is no longer covered. However, it is much less likely than the
still-covered case that ``generateProto`` is not re-generating all files
due to misconfiguration.

zpencer/grpc-java,wrwg/grpc-java,aglne/grpc-java,eonezhang/grpc-java,winstar/grpc-java,louiscryan/grpc-java,wrwg/grpc-java,pieterjanpintens/grpc-java,carl-mastrangelo/grpc-java,aglne/grpc-java,ejona86/grpc-java,LuminateWireless/grpc-java,conchlee/grpc-java,mingfly/grpc-java,zhangkun83/grpc-java,conchlee/grpc-java,fengshao0907/grpc-java,simonhorlick/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,pieterjanpintens/grpc-java,wyxacc/grpc-java,huangsihuan/grpc-java,huangsihuan/grpc-java,louiscryan/grpc-java,vampiregod1996/grpc-java,vampiregod1996/grpc-java,zpencer/grpc-java,fengshao0907/grpc-java,mingfly/grpc-java,vampiregod1996/grpc-java,anuraaga/grpc-java,moujian/grpc-java,jcanizales/grpc-java,madongfly/grpc-java,eamonnmcmanus/grpc-java,aglne/grpc-java,eamonnmcmanus/grpc-java,LuminateWireless/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,ybv/grpc-java,stanley-cheung/grpc-java,LuminateWireless/grpc-java,wyxacc/grpc-java,zpencer/grpc-java,grpc/grpc-java,nmittler/grpc-java,ejona86/grpc-java,eamonnmcmanus/grpc-java,pieterjanpintens/grpc-java,pieterjanpintens/grpc-java,simonhorlick/grpc-java,dapengzhang0/grpc-java,simonhorlick/grpc-java,madongfly/grpc-java,ybv/grpc-java,ybv/grpc-java,louiscryan/grpc-java,elandau/grpc-java,anuraaga/grpc-java,ejona86/grpc-java,nickethier/grpc-java,elandau/grpc-java,fengshao0907/grpc-java,eonezhang/grpc-java,rmichela/grpc-java,jcanizales/grpc-java,eonezhang/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,nmittler/grpc-java,rmichela/grpc-java,stanley-cheung/grpc-java,madongfly/grpc-java,moujian/grpc-java,moujian/grpc-java,dapengzhang0/grpc-java,nickethier/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,rmichela/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,wrwg/grpc-java,simonhorlick/grpc-java,winstar/grpc-java,rmichela/grpc-java,anuraaga/grpc-java,zhangkun83/grpc-java,huangsihuan/grpc-java,wyxacc/grpc-java,nickethier/grpc-java,ejona86/grpc-java,joshuabezaleel/grpc-java,mingfly/grpc-java,zpencer/grpc-java,nmittler/grpc-java,jcanizales/grpc-java,elandau/grpc-java,joshuabezaleel/grpc-java,stanley-cheung/grpc-java,conchlee/grpc-java,winstar/grpc-java,joshuabezaleel/grpc-java,dapengzhang0/grpc-java