What exactly would you like to remove ? If you remove the : detection, then you can no longer open the native emoji selector at all as the only way to open it is inserting : (when you click the emoji button it inserts :).
So if you do want to remove all that, you basically want to stop calling https://github.com/flarum/emoji/blob/master/js/src/forum/addComposerAutocomplete.js
If you fork the emoji extension that's easy, just remove line #9 from index.js https://github.com/flarum/emoji/blob/master/js/src/forum/index.js#L9.
Disabling that function from another extension is a bit difficult though. Lots of code would need to be rewritten. You could completely remove the javascript extender via an other extension, but then you will also lose the emojis in previews.
