Remove -Zkeep-hygiene-data.

It was added way back in #28585 under the name `-Zkeep-mtwt-tables`. The
justification was:

> This is so that the resolution results can be used after analysis,
> potentially for tool support.

There are no uses of significance in the code base, and various Google
searches for both option names (and variants) found nothing of interest.

@petrochenkov says removing this part (and it's only part) of the
hygiene data is dubious. It doesn't seem that big, so let's just keep it
around.

compiler/rustc_interface/src/tests.rs

@@ -679,7 +679,6 @@ fn test_unstable_options_tracking_hash() {
     untracked!(incremental_info, true);
     untracked!(incremental_verify_ich, true);
     untracked!(input_stats, true);
-    untracked!(keep_hygiene_data, true);
     untracked!(link_native_libraries, false);
     untracked!(llvm_time_trace, true);
     untracked!(ls, vec!["all".to_owned()]);