Remove useless comparison since now self.index <= self.len is an invariant

Author: SkiFire13

library/core/src/iter/adapters/zip.rs

@@ -259,7 +259,7 @@ where
             if sz_a != sz_b {
                 let sz_a = self.a.size();
                 if A::MAY_HAVE_SIDE_EFFECT && sz_a > self.len {
-                    for _ in 0..sz_a - cmp::max(self.len, self.index) {
+                    for _ in 0..sz_a - self.len {
                         self.a.next_back();
                     }
                 }