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.

compiler/rustc_interface/src/tests.rs

@@ -843,7 +843,6 @@ 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));