Remove support for old pre_clone_script and post_clone_script configuration settings

Follow-up to #29362 (closed).

With !3713 (merged) we've introduced new configuration settings for "script hooks" that can be executed before and after updating sources is done. The same MR deprecated the old configuration keys pre_clone_script and post_clone_script and introduced a mechanism that is automatically rewriting the configuration file if needed.

With this issue we're going to entirely remove the old configuration settings and the rewriting mechanism.