Revert "Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators."

This reverts commit 3e86cf36b5.

src/liballoc/string.rs

@@ -2385,10 +2385,6 @@ fn next(&mut self) -> Option<char> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
-    #[inline]
-    fn last(mut self) -> Option<char> {
-        self.next_back()
-    }
 }
 
 #[stable(feature = "drain", since = "1.6.0")]