Remove variation selectors from quick reactions
This fixes a regression introduced by the full emoji picker which inserted empty
variation selectors in the thumbs up / down quick reactions. Since this makes
them different characters, it would cause us to not aggregate thumbs from web
vs. mobile together.
Regressed by https://github.com/matrix-org/matrix-react-sdk/pull/3554
Fixes https://github.com/vector-im/riot-web/issues/11335
