Remove unused bridge_exceptions method

The bridge_exceptions method was part of a plan we didn't follow through on. The idea at the time was that we wanted to transparently re-raise gitaly-ruby exceptions in gitlab-ce. We never built the gitlab-ce part and we changed our approach to exceptions; we now use mostly use gRPC code FailedPrecondition if we want to signal an "expected exception".

This bridge_exceptions thing was just boilerplate that everybody keeps adding in new gitaly-ruby code.

Closes #608 (closed) #698 (closed)