drm/i915/cx0: remove the unused intel_is_c10phy()

The intel_is_c10phy() is now unused. Remove.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/486ad2832c567ae491726c6c0cd7144e14469a2f.1710949619.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>