Remove highlighting of spans on -Zteach

`-Zteach` is perma-unstable, barely used, the highlighting logic buggy and the flag being passed around is tech-debt. We should likely remove `-Zteach` in its entirely.

compiler/rustc_errors/src/emitter.rs

@@ -616,7 +616,6 @@ pub struct HumanEmitter {
     #[setters(skip)]
     fallback_bundle: LazyFallbackBundle,
     short_message: bool,
-    teach: bool,
     ui_testing: bool,
     ignored_directories_in_source_blocks: Vec<String>,
     diagnostic_width: Option<usize>,
@@ -642,7 +641,6 @@ impl HumanEmitter {
             fluent_bundle: None,
             fallback_bundle,
             short_message: false,
-            teach: false,
             ui_testing: false,
             ignored_directories_in_source_blocks: Vec::new(),
             diagnostic_width: None,
@@ -1044,15 +1042,6 @@ impl HumanEmitter {
                         underline.style,
                     );
                 }
-                _ if self.teach => {
-                    buffer.set_style_range(
-                        line_offset,
-                        (code_offset + annotation.start_col.display).saturating_sub(left),
-                        (code_offset + annotation.end_col.display).saturating_sub(left),
-                        underline.style,
-                        annotation.is_primary,
-                    );
-                }
                 _ => {}
             }
         }

compiler/rustc_session/src/session.rs

@@ -930,7 +930,6 @@ fn default_emitter(
                     .fluent_bundle(bundle)
                     .sm(source_map)
                     .short_message(short)
-                    .teach(sopts.unstable_opts.teach)
                     .diagnostic_width(sopts.diagnostic_width)
                     .macro_backtrace(macro_backtrace)
                     .track_diagnostics(track_diagnostics)

src/librustdoc/core.rs

@@ -160,7 +160,6 @@ pub(crate) fn new_dcx(
                 HumanEmitter::new(stderr_destination(color_config), fallback_bundle)
                     .sm(source_map.map(|sm| sm as _))
                     .short_message(short)
-                    .teach(unstable_opts.teach)
                     .diagnostic_width(diagnostic_width)
                     .track_diagnostics(unstable_opts.track_diagnostics)
                     .theme(if let HumanReadableErrorType::Unicode = kind {