drm/i915/selftests: Remove flush_scheduled_work() from live_execlists

There are ongoing efforts to remove usages of flush_scheduled_work() from
drivers in order to avoid several cases of potentential problems when
flushing is done from certain contexts.

Remove the call from the live_execlists selftest. Its purpose was to be
thorough and sync with the execlists capture state handling, but that is
not strictly required for the test to function and can be removed.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220630125716.50835-1-tvrtko.ursulin@linux.intel.com