Revert GitLab mail delivery workaround
In #40873 (closed) we implemented an emergency workaround to fix mail delivery in GitLab after the upgrade to 15.3 last August.
The underlying issue, described in GitLab ticket #371716 is reportedly fixed in 15.6, and there's at least one report confirming the fix on Debian+Omnibus.
Hence we should revert the workaround and switch back to the default delivery method, as this will bring us back closer to the default GitLab config and allow us to benefit from the fixes that justified the change to the webhook
mail delivery method in the first place.