Feature flag: removal of filter_shas_with_signatures_go

What does this MR do?

Previously gRPC method FilterShasWithSignatures was part of Gitaly-Ruby, that means it was implemented in Ruby and used Rugged to interact with Git. Because of moving out from Ruby to Go implementation the method was implemented in Go and a feature flag was added to switch between implementations. After some time in production with enabled feature flag (use of Go implementation) it was decided to remove Gitaly-Ruby implementation and a feature-flag. And as of now the flag is not used anymore by Gitaly and no any other dependency exists. So this MR removes feature flag filter_shas_with_signatures_go as it redundant.

Please see related issue: gitaly#2268 (closed)

Availability and Testing

Nothing to be broken. The flag is not used.