Fix build with rustc 1.2.0-nightly (20d23d8 2015-06-18)

In rust-lang/rust#26347, MacroFormat was
renamed to ExpnFormat. MacroFormat wasn't being used in
src/mut_mut.rs, so I removed it.

Author: zmbush

src/mut_mut.rs

@@ -2,7 +2,7 @@ use syntax::ptr::P;
 use syntax::ast::*;
 use rustc::lint::{Context, LintPass, LintArray, Lint};
 use rustc::middle::ty::{expr_ty, TypeVariants, mt, TyRef};
-use syntax::codemap::{BytePos, ExpnInfo, MacroFormat, Span};
+use syntax::codemap::{BytePos, ExpnInfo, Span};
 use utils::in_macro;
 
 declare_lint!(pub MUT_MUT, Warn,