Remove gitaly_always_log_feature_flags flag

In Add feature flag field to Gitaly logs (!5054 - merged), we introduced feature flag logging as a part of gRPC logging middleware. That was a retry attempt, because we blew up our centralized logging system due to huge volume of feature flag logs. As a result, we attach feature_flag files to failed RPCs only. We don't include them in successful RPCs unless gitaly_always_log_feature_flags flag is on. After turned on the flag on production, there was no impact on our logging system.

For further information about the rollout, please visit [Feature flag] gitaly_always_log_feature_flags (#4627 - closed).