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.
I think this can safely be removed.

Author: nnethercote

compiler/rustc_ast_lowering/src/lib.rs

@@ -444,9 +444,7 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
     sess.time("drop_ast", || drop(krate));
 
     // Discard hygiene data, which isn't required after lowering to HIR.
-    if !sess.opts.unstable_opts.keep_hygiene_data {
-        rustc_span::hygiene::clear_syntax_context_map();
-    }
+    rustc_span::hygiene::clear_syntax_context_map();
 
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =

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()]);

compiler/rustc_session/src/options.rs

@@ -1622,8 +1622,6 @@ options! {
          `=skip-entry`
          `=skip-exit`
          Multiple options can be combined with commas."),
-    keep_hygiene_data: bool = (false, parse_bool, [UNTRACKED],
-        "keep hygiene data after analysis (default: no)"),
     layout_seed: Option<u64> = (None, parse_opt_number, [TRACKED],
         "seed layout randomization"),
     link_directives: bool = (true, parse_bool, [TRACKED],