Remove unnecessary optimization FIXME

As the comment suggests, the return value of frameless calls is rarely not used.
When it isn't used (and isn't refcounted) the FREE is already elided by the
optimizer.

Closes GH-20819

Zend/zend_compile.c

@@ -816,8 +816,6 @@ static void zend_do_free(znode *op1) /* {{{ */
 			} else {
 				/* Frameless calls usually use the return value, so always emit a free. This should be
 				 * faster than checking RETURN_VALUE_USED inside the handler. */
-				// FIXME: We may actually look at the function signature to determine whether a free
-				// is necessary.
 				zend_emit_op(NULL, ZEND_FREE, op1, NULL);
 			}
 		} else {