Rollup merge of #133847 - nnethercote:rm-Z-show-span, r=compiler-errors

Remove `-Zshow-span`.

It's very old (added in #12087). It's strange, and it's not clear what its use cases are. It only works with the crate root file because it runs before expansion. I suspect it won't be missed.

r? `@estebank`

compiler/rustc_interface/src/tests.rs

@@ -844,7 +844,6 @@ fn test_unstable_options_tracking_hash() {
     tracked!(sanitizer_recover, SanitizerSet::ADDRESS);
     tracked!(saturating_float_casts, Some(true));
     tracked!(share_generics, Some(true));
-    tracked!(show_span, Some(String::from("abc")));
     tracked!(simulate_remapped_rust_src_base, Some(PathBuf::from("/rustc/abc")));
     tracked!(small_data_threshold, Some(16));
     tracked!(split_lto_unit, Some(true));