drm/modes: Kill DRM_MODE_TYPE_CLOCK_CRTC_C define

No idea what the DRM_MODE_TYPE_CLOCK_CRTC_C define is supposed to
achieve. Totally unused so kill if off.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-8-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>