drm/i915/bios: remove unused, obsolete VBT definitions

We've carried this baggage for more than a decade. Time to let it go.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/00058e6507aa7a62fce1d2a6de223cbbfabb204b.1559308269.git.jani.nikula@intel.com