This removes unnecessary gRPC source and binaries that were bundled with
the ARM package. Prior to this change, the `grpc-1.30.2` gem was taking
220 MB of space. We can remove a number of files in the
`/opt/gitlab/embedded/lib/ruby/gems/2.7.0/gems/grpc-1.30.2` directory:

1. `/include`: 400K
2. `/src/core`: 9.3 MB
3. `/src/ruby/ext`: 196.1 MB
2. `/third_party`: 12.1 MB

These directories do not include any Ruby code, so it is safe to delete
them. `/src/ruby/ext` contains mostly C source and headers, along with
compiled objects.

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5661