git: Remove support for the Ruby hooks directory

Historically, our Git hooks were located in the Ruby directory as a set of wrapper scripts which derive the actual location of the gitaly-hooks binary via the GITALY_BIN_DIR environment variable. This has been replaced with a new mode though, where we instead create a temporary directory which contains symlinks for all of the hooks which directly point to gitaly-hooks. This is done both to remove the indirection via another shell script, and more importantly to get rid of our dependency on the Ruby directory.

This new mode has been rolled out as default-enabled in 14.8, so we can now remove support for the old Ruby hooks directory in this release.

Changelog: removed

Closes #4006 (closed)