drm/doc: Drop "content type" from the legacy kms property table

We want new stuff documented in more verbose form, this table is
deprecated. "content type" is already documented properly.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 50525c33 ("drm: content-type property for HDMI connector")
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326090555.5969-1-daniel.vetter@ffwll.ch