As title. Most logic will be in google3, see cl/368529309.

We added "_xds" suffix Bazel rules, because we hope to have a clean separation between specially treated targets and normal targets. However, grpcpp_csds is going to be depended by many tests and binaries, existing solution requires people to remember changing the normal rule to "_xds" ones, WHENEVER the target transitively depends on grpcpp_csds. This is very error-prone.

Besides, Bazel rules grpc_cc_library/grpc_cc_test are exempted from strict headers check: https://source.corp.google.com/piper///depot/google3/java/com/google/devtools/build/lib/view/cpp/LayeringCheckAllowlists.java?q=grpc_cc_library. The new "_xds" Bazel rules ain't, and will trigger following error:

ERROR: /google/src/cloud/lidiz/github-sync-20210414144431/google3/third_party/grpc/BUILD:2647:20: in cc_library rule //third_party/grpc:grpcpp_admin: C++ rules should use 'strict' hdrs_check (not 'loose' or 'warn').The hdrs_check='strict' ensures that all #included files must be explicitly declared somewhere in the hdrs or srcs of the rules providing the libraries or the rules containing the including source. Please remove 'hdrs_check' attribute from this rule, as well as the 'default_hdrs_check' attribute from the package, or set them to 'strict'. If you think your use case qualifies for an exception, please see go/blaze-strict-layering-exception

So, this PR:

• Treat DISABLED_XDS_PROTO_IN_CC as GRPC_NO_XDS in admin services
• Simplified the preprocessing logic in admin_services_end2end_test
• Remove all _xds Bazel rules (internally, existing logics are merged into normal Bazel rules)

This change is