drm/i915: Remove unused i915_flip tracepoints

The i915_flip* tracepoints are no longer in use since the removal of CS
flip in commit 8b5d27b9 ("drm/i915: Remove intel_flip_work
infrastructure")

References: 8b5d27b9 ("drm/i915: Remove intel_flip_work infrastructure")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508151552.31024-1-chris@chris-wilson.co.uk