Remove flaky test (#3098)
I introduced a flaky test to confirm that `MegolmEncryption#prepareToEncrypt`
didn't block the main thread too much, but it turns out that, when run in
varying environments, it tends to fail.
The same behavior is guaranteed by the following cancellation test - if the
thread is blocked, it can't be cancelled.
Signed-off-by: Clark Fischer <clark.fischer@gmail.com>
Signed-off-by: Clark Fischer <clark.fischer@gmail.com>
