Revert "Highlight conflicting param-env candidates"

This reverts commit 08135254dc.

compiler/rustc_middle/src/traits/mod.rs

@@ -541,17 +541,9 @@ 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:
 ///