drm/i915/selftests: Remove live_suppress_wait_preempt

With the removal of the internal wait-priority boosting, we can also
remove the selftest to ensure that those waits were being suppressed
from causing preemptions.

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/20200607222108.14401-4-chris@chris-wilson.co.uk