descriptor.proto uses proto2 syntax, which is not ready for external
usage. However, some proto3 files import descriptor.proto and cannot be
used. In this PR, all references (We cheated by only removing
extensions, which is enough for now. User should avoid using messages
defined in descriptor.proto as field type.) to content in
descriptor.proto are removed from generated files. Those that import
descriptor.proto can be used like other proto files.