rustc: Remove #[rustc_synthetic]

This function parameter attribute was introduced in https://github.com/rust-lang/rust/pull/44866 as an intermediate step in implementing `impl Trait`, it's not necessary or used anywhere by itself.

compiler/rustc_hir/src/hir.rs

@@ -504,7 +504,7 @@ pub enum GenericParamKind<'hir> {
     },
     Type {
         default: Option<&'hir Ty<'hir>>,
-        synthetic: Option<SyntheticTyParamKind>,
+        synthetic: bool,
     },
     Const {
         ty: &'hir Ty<'hir>,
@@ -577,16 +577,6 @@ impl Generics<'hir> {
     }
 }
 
-/// Synthetic type parameters are converted to another form during lowering; this allows
-/// us to track the original form they had, and is useful for error messages.
-#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, Debug)]
-#[derive(HashStable_Generic)]
-pub enum SyntheticTyParamKind {
-    ImplTrait,
-    // Created by the `#[rustc_synthetic]` attribute.
-    FromAttr,
-}
-
 /// A where-clause in a definition.
 #[derive(Debug, HashStable_Generic)]
 pub struct WhereClause<'hir> {