This commit removes the following gogoproto extensions;
- gogoproto.nullable
- gogoproto.customename
- gogoproto.unmarshaller_all
- gogoproto.stringer_all
- gogoproto.sizer_all
- gogoproto.marshaler_all
- gogoproto.goproto_unregonized_all
- gogoproto.goproto_stringer_all
- gogoproto.goproto_getters_all
None of them are supported by Google's toolchain (see #6564).
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
