Language packs: Remove translations when deleting a theme or a plugin.

This is for translation files in WP_LANG_DIR which are installed through a language pack.
Change `wp_get_installed_translations()` to only return a translation if the .mo file also exists.

fixes #29860.

git-svn-id: https://develop.svn.wordpress.org/trunk@29856 602fd350-edb4-49c9-b593-d223f7449a82