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_ast_lowering/src/lib.rs

@@ -443,11 +443,6 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
     drop(ast_index);
     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();
-    }
-
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =
         if tcx.needs_crate_hash() { Some(compute_hir_hash(tcx, &owners)) } else { None };

compiler/rustc_interface/src/tests.rs

@@ -679,7 +679,6 @@ macro_rules! untracked {
     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 @@ pub(crate) fn parse_inlining_threshold(slot: &mut InliningThreshold, v: Option<&
          `=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],

compiler/rustc_span/src/hygiene.rs

@@ -569,10 +569,6 @@ fn apply_mark_internal(
     }
 }
 
-pub fn clear_syntax_context_map() {
-    HygieneData::with(|data| data.syntax_context_map = FxHashMap::default());
-}
-
 pub fn walk_chain(span: Span, to: SyntaxContext) -> Span {
     HygieneData::with(|data| data.walk_chain(span, to))
 }