Best practice for Bazel builds gives one rule per target, and producing
multiple targets conflicts with this.

Short term: define macros, and common libraries that can eliminate the
duplication.

Longer term: eliminate the existing plugin registration mechanism, and
replace it with a C++ static initialization in appropriate files. This
will allow grpc to be layered strictly atop grpc_unsecure (and similarly
grpc++ atop grpc++_unsecure and grpc).

This reverts commit 79759fea, reversing
changes made to dc36f4df.