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_session/src/options.rs

@@ -1553,9 +1553,6 @@ options! {
     dep_info_omit_d_target: bool = (false, parse_bool, [TRACKED],
         "in dep-info output, omit targets for tracking dependencies of the dep-info files \
         themselves (default: no)"),
-    dont_buffer_diagnostics: bool = (false, parse_bool, [UNTRACKED],
-        "emit diagnostics rather than buffering (breaks NLL error downgrading, sorting) \
-        (default: no)"),
     dual_proc_macros: bool = (false, parse_bool, [TRACKED],
         "load proc macros for both target and host, but only link to the target (default: no)"),
     dump_dep_graph: bool = (false, parse_bool, [UNTRACKED],