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

drivers/gpu/drm/i915/display/i9xx_wm.h

@@ -12,7 +12,6 @@ struct drm_i915_private;
 struct intel_crtc_state;
 struct intel_plane_state;
 
-int ilk_wm_max_level(const struct drm_i915_private *i915);
 bool ilk_disable_lp_wm(struct drm_i915_private *i915);
 void ilk_wm_sanitize(struct drm_i915_private *i915);
 bool intel_set_memory_cxsr(struct drm_i915_private *i915, bool enable);