drm/i915/selftests: Drop vestigal timeslicing assert

Since the semaphore interrupt may cause us to yield the timeslice
immediately, we may cancel the timer before we notice the submission is
complete. The assertion is no longer valid due to the race with the
interrupt.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200407222625.15542-1-chris@chris-wilson.co.uk