Cleanup: Remove unnecessary use of uiBut::a2

The color luminosity was written to this value, but never read
anywhere. uiBut::a2 is only read in three more places, none of
which could reasonably use this value.

source/blender/editors/interface/interface_templates.cc

@@ -5611,7 +5611,6 @@ void uiTemplateColorPicker(uiLayout *layout,
   if (lock_luminosity) {
     float color[4]; /* in case of alpha */
     RNA_property_float_get_array(ptr, prop, color);
-    but->a2 = len_v3(color);
     cpicker->luminosity_lock_value = len_v3(color);
   }