Remove not used mod

`mir_stats` mod has not been used since c1ff104.

Author: yui-knk

src/librustc_passes/lib.rs

@@ -41,7 +41,6 @@ pub mod ast_validation;
 pub mod rvalue_promotion;
 pub mod hir_stats;
 pub mod loops;
-mod mir_stats;
 
 __build_diagnostic_array! { librustc_passes, DIAGNOSTICS }