um: Fix time-travel mode

Since the time-travel rework, basic time-travel mode hasn't worked
properly, but there's no longer a need for this WARN_ON() so just
remove it and thereby fix things.

Cc: stable@vger.kernel.org
Fixes: 4b786e24 ("um: time-travel: Rewrite as an event scheduler")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>