operations: Wire up AllowConflicts handling for Go

Originally, handling of the "AllowConflicts" parameter for the UserMergeToRef RPC was only implemented in Ruby and not in Go because efforts to port to Go and to implement this new flag crossed. This has changed with the previous commit so that we can now switch over to fully use the Go port instead of the Ruby code in all cases.

Note that this directly removes the Ruby codepath without any kind of feature flag. This is done because first, the UserMergeToRef RPC has been tested already and rolled out by default. And second, all callers in GitLab Rails which make use of "AllowConflicts" are already hidden behind the "display_merge_conflicts_in_diff" feature flag which is currently disabled by default. Introducing another feature flag for this parameter thus doesn't seem necessary.