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_ast_passes/src/errors.rs

@@ -779,14 +779,6 @@ pub(crate) struct IncompatibleFeatures {
     pub f2: Symbol,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_show_span)]
-pub(crate) struct ShowSpan {
-    #[primary_span]
-    pub span: Span,
-    pub msg: &'static str,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_negative_bound_not_supported)]
 pub(crate) struct NegativeBoundUnsupported {