etcupdate: Remove the old pre-world tree when rotating for -p.

This fixes a bug in an earlier change to move tree rotation to
the end of the update where the step to make room for the new
preworld tree was deleting the old "current" tree instead of
the old "preworld" tree.

Reported by:	olivier, dhw
Fixes:		0611aec3cf
MFC after:	2 weeks

usr.sbin/etcupdate/etcupdate.sh

@@ -1644,7 +1644,7 @@ EOF
 			if [ -z "$preworld" ]; then
 				panic "New tree should be rotated to old"
 			fi
-			if ! remove_tree $old; then
+			if ! remove_tree $new; then
 				echo "Unable to remove previous pre-world tree."
 				exit 1
 			fi