Always enable subprocess logging over pipes

Child processes spawned by Gitaly, gitaly-hooks and gitaly-lfs-smudge, have so far been writing their log output into separate files. In Implement subprocess logging via pipes (!7102 - merged), we implemented a logger that passes the logs from the child processes to the main gitaly process over a pipe. This way the logs from the child processes can be output in the same location as the logs of Gitaly itself. This eases log management as we no longer have multiple log files to handle and all of the logs can be printed out through the standard output.

The new logger was put behind a feature flag which has been successfully enabled on production. As we don't really expect any issues with the change, this commit removes the flag directly. Subprocess logging over pipes is now always enabled.

Related to #6222 (closed)