Move deny(warnings) into rustbuild

This permits easier iteration without having to worry about warnings
being denied.

Fixes #49517

src/liballoc/tests/lib.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(allocator_api)]
 #![feature(alloc_system)]
 #![feature(attr_literals)]