Auto merge of #83491 - jyn514:remove-pretty, r=pnkfelix

Remove unstable `--pretty` flag

It doesn't do anything `--unpretty` doesn't, and due to a bug, also
didn't show up in `--help`. I don't think there's any reason to keep it
around, I haven't seen anyone using it.

Closes https://github.com/rust-lang/rust/issues/36473.

compiler/rustc_session/src/options.rs

@@ -1308,7 +1308,7 @@ options! {
         "take the brakes off const evaluation. NOTE: this is unsound (default: no)"),
     unpretty: Option<String> = (None, parse_unpretty, [UNTRACKED],
         "present the input source, unstable (and less-pretty) variants;
-        valid types are any of the types for `--pretty`, as well as:
+        `normal`, `identified`,
         `expanded`, `expanded,identified`,
         `expanded,hygiene` (with internal representations),
         `everybody_loops` (all function bodies replaced with `loop {}`),