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/ast.rs

@@ -529,9 +529,6 @@ pub enum Expr_ {
     ExprAgain(Option<Ident>),
     ExprRet(Option<@Expr>),
 
-    /// Gets the log level for the enclosing module
-    ExprLogLevel,
-
     ExprInlineAsm(InlineAsm),
 
     ExprMac(Mac),