Remove Firefox transitioncancel entry from Most Wanted list

The transitioncancel event was shipped in Firefox 53 🎉
https://bugzilla.mozilla.org/show_bug.cgi?id=1264125

_data/browser-features.yml

@@ -58,16 +58,6 @@
   origin: >
     Bootstrap#20143
 
--
-  browser: >
-    Firefox
-  summary: >
-    Fire a [`transitioncancel` event](https://developer.mozilla.org/en-US/docs/Web/Events/transitioncancel) when a CSS transition is canceled
-  upstream_bug: >
-    Mozilla#1264125
-  origin: >
-    Mozilla#1182856
-
 -
   browser: >
     Firefox