drm/i915: Remove defunct trace points

trace_i915_gem_evict_everything and trace_i915_gem_ring_flush stopped
being used when their parent functions were removed.

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/20171003125055.11370-2-chris@chris-wilson.co.uk