LU-18956 mgc: Remove llog_config_lock

LU-18956 mgc: Remove llog_config_lock

This was alredy previously removed as nothing is using it,
but accidentally got resurrected by LU-11185 patch.

Remove it again.

Change-Id: Ie68c54d3401473c0ddb095c49ce753a7b94063ca
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Test-Parameters: trivial
Fixes: 0ad54d5977736 ("LU-11185 mgc: config lock leak")
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58992
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>