Allow unused extern crate again

This is a partial revert of rust-lang#42588. There is a usability concern
reported in rust-lang#44294 that was not considered in the discussion of the PR,
so I would like to back this out of 1.21. As is, I think users would
have a worse and more confusing experience with this lint enabled by
default. We can re-enabled once there are better diagnostics or the case
in rust-lang#44294 does not trigger the lint.

src/librustc/lint/builtin.rs

@@ -30,7 +30,7 @@ declare_lint! {
 
 declare_lint! {
     pub UNUSED_EXTERN_CRATES,
-    Warn,
+    Allow,
     "extern crates that are never used"
 }