tree.def: Remove outdated comment on SAD_EXPR

While looking at PR108692, I've noticed SAD_EXPR comment mentions that
WIDEN_MINUS_EXPR is missing, which is not true anymore since r11-5160.

The following patch just removes that part of the comment.

2023-02-08  Jakub Jelinek  <jakub@redhat.com>

	* tree.def (SAD_EXPR): Remove outdated comment about missing
	WIDEN_MINUS_EXPR.