Remove some now-unnecessary impls

Author: oli-obk

compiler/rustc_metadata/src/rmeta/table.rs

@@ -196,13 +196,6 @@ fixed_size_enum! {
     }
 }
 
-fixed_size_enum! {
-    hir::Constness {
-        ( NotConst )
-        ( Const    )
-    }
-}
-
 fixed_size_enum! {
     hir::Defaultness {
         ( Final                        )
@@ -218,13 +211,6 @@ fixed_size_enum! {
     }
 }
 
-fixed_size_enum! {
-    ty::Asyncness {
-        ( Yes )
-        ( No  )
-    }
-}
-
 fixed_size_enum! {
     hir::CoroutineKind {
         ( Coroutine(hir::Movability::Movable)                                          )