Workaround Qt5 bug that fails to remove separator at end of File/Help menus (#3350)

Prevent dangling separator at end of menus on macOS

Per https://bugreports.qt.io/browse/QTBUG-40071
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f