Remove impl Foo for .. in favor of auto trait Foo

No longer parse it.
Remove AutoTrait variant from AST and HIR.
Remove backwards compatibility lint.
Remove coherence checks, they make no sense for the new syntax.
Remove from rustdoc.

src/libsyntax/ast.rs

@@ -2022,10 +2022,6 @@ pub enum ItemKind {
     ///
     /// E.g. `trait Foo = Bar + Quux;`
     TraitAlias(Generics, TyParamBounds),
-    /// Auto trait implementation.
-    ///
-    /// E.g. `impl Trait for .. {}` or `impl<T> Trait<T> for .. {}`
-    AutoImpl(Unsafety, TraitRef),
     /// An implementation.
     ///
     /// E.g. `impl<A> Foo<A> { .. }` or `impl<A> Trait for Foo<A> { .. }`
@@ -2064,8 +2060,7 @@ impl ItemKind {
             ItemKind::TraitAlias(..) => "trait alias",
             ItemKind::Mac(..) |
             ItemKind::MacroDef(..) |
-            ItemKind::Impl(..) |
-            ItemKind::AutoImpl(..) => "item"
+            ItemKind::Impl(..) => "item"
         }
     }
 }