Extract grpcio plugins (#13896)

stubs/grpcio/METADATA.toml

@@ -1,15 +1,6 @@
 version = "1.*"
 upstream_repository = "https://github.com/grpc/grpc"
 partial_stub = true
-requires = [
-    "types-protobuf",
-]
 
 [tool.stubtest]
 ignore_missing_stub = true
-stubtest_requirements = [
-    "grpcio-channelz",
-    "grpcio-health-checking",
-    "grpcio-reflection",
-    "grpcio-status",
-]