Remove unused imports and dependency in .proto files

Our proto files included imports which had gone unused. The compiler detected this, and emitted warnings. Removing the imports removed these lines.

The second commit removes another warning, one of the two

[libprotobuf WARNING google/protobuf/compiler/ruby/ruby_generator.cc:509] Omitting proto2 dependency 'google/protobuf/descriptor.proto' from proto3 output file 'lint_pb.rb' because we don't support proto2 and no proto2 types from that file are being used.

Which again, is because of removing one dependency.