Rollup merge of #99290 - compiler-errors:revert-98794, r=lcnr

Revert "Highlight conflicting param-env candidates"

This reverts #98794, commit 08135254dc.

Seems to have caused an incremental compilation bug. The root cause of the incr comp bug is somewhat unrelated but is triggered by this PR, so I don't feel comfortable with having this PR in the codebase until it can be investigated further. Fixes #99233.

compiler/rustc_middle/src/traits/mod.rs

@@ -542,17 +542,9 @@ pub enum SelectionError<'tcx> {
     ErrorReporting,
     /// Multiple applicable `impl`s where found. The `DefId`s correspond to
     /// all the `impl`s' Items.
-    Ambiguous(Vec<AmbiguousSelection>),
+    Ambiguous(Vec<DefId>),
 }
 
-#[derive(Copy, Clone, Debug)]
-pub enum AmbiguousSelection {
-    Impl(DefId),
-    ParamEnv(Span),
-}
-
-TrivialTypeTraversalAndLiftImpls! { AmbiguousSelection, }
-
 /// When performing resolution, it is typically the case that there
 /// can be one of three outcomes:
 ///