operations: Always return structured errors from Git2Go merges
In commit 84bd2721 (gitaly-git2go: Return structured conflict error on merge conflict, 2021-08-26), we have changed gitaly-git2go's merge subcommand to support structured errors. This allows us to return more information why a merge failed, e.g. by returning the set of conflicting files to the caller.
Drop the feature flag which guards this code and thus always enable the use of structured errors.
Closes #3756 (closed)
Edited by Patrick Steinhardt