Remove -Zdont-buffer-diagnostics.

It was added in #54232. It seems like it was aimed at NLL development,
which is well in the past. Also, it looks like `-Ztreat-err-as-bug` can
be used to achieve the same effect. So it doesn't seem necessary.

compiler/rustc_interface/src/tests.rs

@@ -659,7 +659,6 @@ fn test_unstable_options_tracking_hash() {
     // tidy-alphabetical-start
     untracked!(assert_incr_state, Some(String::from("loaded")));
     untracked!(deduplicate_diagnostics, false);
-    untracked!(dont_buffer_diagnostics, true);
     untracked!(dump_dep_graph, true);
     untracked!(dump_mir, Some(String::from("abc")));
     untracked!(dump_mir_dataflow, true);