drm/i915/wm: remove stale and unused ilk_wm_max_level() declaration

The declaration was removed earlier, but got accidentally resurrected in
i915xx_wm.[ch] refactoring. Remove harder.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230419115449.422214-1-jani.nikula@intel.com