Remove commented out code from Button

Some hangover from the work to migrate `disabled` out of `Button` and into
`Widget`, that I forgot to remove.

src/textual/widget.py

@@ -1177,6 +1177,11 @@ class Widget(DOMNode):
         """
         return self.virtual_region.grow(self.styles.margin)
 
+    @property
+    def focusable(self) -> bool:
+        """Can this widget currently receive focus?"""
+        return self.can_focus and not self.disabled
+
     @property
     def focusable_children(self) -> list[Widget]:
         """Get the children which may be focused.