Revert "Switch back non_local_definitions lint to allow-by-default"

This reverts commit 0c0dfb88ee.

compiler/rustc_lint/src/non_local_def.rs

@@ -49,7 +49,7 @@ declare_lint! {
     /// All nested bodies (functions, enum discriminant, array length, consts) (expect for
     /// `const _: Ty = { ... }` in top-level module, which is still undecided) are checked.
     pub NON_LOCAL_DEFINITIONS,
-    Allow,
+    Warn,
     "checks for non-local definitions",
     report_in_external_macro
 }