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_span/src/hygiene.rs

@@ -569,10 +569,6 @@ impl HygieneData {
     }
 }
 
-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))
 }