Remove deprecated flag for lychee

The --exclude-mail flag is deprecated and no longer needed

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2759>

.gitlab-ci.yml

@@ -144,7 +144,6 @@ find_broken_links:
       --cache
       --no-progress
       --exclude-all-private
-      --exclude-mail
       --exclude 'demo\.funkwhale\.audio'
       --exclude 'nginx\.com'
       --exclude-path 'docs/_templates/'