Fix: buttons whose property contains an 'owner_id' ignore rna undo check

Introduced in rBce462fa1 but harmless since curretly only `StructRNA`
without `owner_id` have the `STRUCT_UNDO` flag cleared.

So this commit does not bring any functional changes but it will be
useful for {D10695}.

source/blender/editors/interface/interface.c

@@ -1038,7 +1038,6 @@ static bool ui_but_is_rna_undo(const uiBut *but)
     if (ID_CHECK_UNDO(id) == false) {
       return false;
     }
-    return true;
   }
   if (but->rnapoin.type && !RNA_struct_undo_check(but->rnapoin.type)) {
     return false;

source/blender/makesrna/RNA_types.h

@@ -619,7 +619,7 @@ typedef enum StructFlag {
   /** Indicates that this struct is an ID struct, and to use reference-counting. */
   STRUCT_ID = (1 << 0),
   STRUCT_ID_REFCOUNT = (1 << 1),
-  /** defaults on, clear for user preferences and similar */
+  /** defaults on, indicates when changes in members of a StructRNA should trigger undo steps. */
   STRUCT_UNDO = (1 << 2),
 
   /* internal flags */