Remove not used option

`mir_stats` has not been used since 2b32cb9.

Author: yui-knk

src/librustc/session/config.rs

@@ -1286,8 +1286,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "print some performance-related statistics"),
     hir_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some statistics about AST and HIR"),
-    mir_stats: bool = (false, parse_bool, [UNTRACKED],
-        "print some statistics about MIR"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],
         "encode MIR of all functions into the crate metadata"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],