Disable event encryption for reactions
For reactions, there is a very little gained by encrypting the entire event, as
relation data is already kept in the clear. Event encryption for a reaction
effectively only obscures the event type, but the purpose is still obvious from
the relation data, so nothing is really gained. It also causes quite a few
problems, such as:
* triggers notifications via default push rules
* prevents server-side bundling for reactions
The reaction key / content / emoji value does warrant encrypting, but this will
be handled separately by encrypting just this value.
See https://github.com/matrix-org/matrix-doc/pull/1849#pullrequestreview-248763642
Fixes https://github.com/vector-im/riot-web/issues/10130
