rustc: Remove compiler support for __log_level()

This commit removes all internal support for the previously used __log_level()
expression. The logging subsystem was previously modified to not rely on this
magical expression. This also removes the only other function to use the
module_data map in trans, decl_gc_metadata. It appears that this is an ancient
function from a GC only used long ago.

This does not remove the crate map entirely, as libgreen still uses it to hook
in to the event loop provided by libgreen.

src/libsyntax/ext/base.rs

@@ -120,7 +120,7 @@ impl MacResult {
     pub fn raw_dummy_expr(sp: codemap::Span) -> @ast::Expr {
         @ast::Expr {
             id: ast::DUMMY_NODE_ID,
-            node: ast::ExprLogLevel,
+            node: ast::ExprTup(Vec::new()),
             span: sp
         }
     }