Remove MIR unsafe check

This also remove safety information from MIR.

compiler/rustc_session/src/options.rs

@@ -1950,8 +1950,6 @@ written to standard error output)"),
     #[rustc_lint_opt_deny_field_access("use `Session::lto` instead of this field")]
     thinlto: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "enable ThinLTO when possible"),
-    thir_unsafeck: bool = (true, parse_bool, [TRACKED],
-        "use the THIR unsafety checker (default: yes)"),
     /// We default to 1 here since we want to behave like
     /// a sequential compiler for now. This'll likely be adjusted
     /// in the future. Note that -Zthreads=0 is the way to get