drm/vblank: Remove DRM_VBLANKTIME_IN_VBLANK

The core code doesn't care at all about this, it's entirely dead.

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322083617.13361-11-daniel.vetter@ffwll.ch