Bug 1883775: remove defaults/pref when it is empty r=bytesized

This will generate an `rmdir` instruction, which will remove the directory if empty.

This is useful as a basic cleanup measure, but it will also fix some of the errors we've seen in our update verify tests (see bug 1883123).

Differential Revision: https://phabricator.services.mozilla.com/D203803

Author: bhearsum

browser/installer/removed-files.in

@@ -77,6 +77,7 @@
 #ifdef XP_MACOSX
 @DIR_RESOURCES@defaults/pref/channel-prefs.js
 @DIR_RESOURCES@defaults/pref/
+@DIR_RESOURCES@defaults/
 #endif
 
 # update-settings.ini has been removed on macOS.