Remove gitaly_pack_objects_limiting_remote_ip feature flag

We've implemented a feature flag that manages the pack-objects concurrency limit based on remote IP. After successful testing on GitLab.com production, we're now removing this feature flag. Afterward, the concurrency limit for pack-objects command invocation is always enabled. The default configuration is 200 per IP per node. The loopback IP is excluded from the limiter. Requests exceeding this limit are forced to be queued. They can stay in the queue as long as they want.

This MR also removes some environment variables which were introduced to boost the development speed.