Constraints: Mark proxy lcoal as not editable

This can not work reliably anyway.

source/blender/makesrna/intern/rna_constraint.c

@@ -2549,6 +2549,7 @@ void RNA_def_constraint(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Active", "Constraint is the one being edited");
 
 	prop = RNA_def_property(srna, "is_proxy_local", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", CONSTRAINT_PROXY_LOCAL);
 	RNA_def_property_ui_text(prop, "Proxy Local",
 	                         "Constraint was added in this proxy instance (i.e. did not belong to source Armature)");