deprecate theme_aware_modal-prompts.css

These are already styled based on theme in recent Firefox versions

chrome/theme_aware_modal_prompts.css

@@ -1,6 +1,8 @@
 /* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/theme_aware_modal_prompts.css made available under Mozilla Public License v. 2.0
 See the above repository for updates as well as full license text. */
 
+/* DEPRECATED */
+
 /* Makes the popup of alert() prompt() etc. respept theme colors as well as be slightly differently styled */
 
 tabmodalprompt >.spacer-top{ display: none }