Stop publishing gitaly/proto as separate go module

Fixes #1839 (closed)

Related to #1761 (closed)

The current situation is broken, it does not import cleanly with go get because gitaly/go.mod relies on a bogus version of the gitaly/proto go module.

This change makes everything simpler. Our downstream consumers rely on gitaly/client anyway so there is no point in publishing gitaly/proto separately.