Dead suggestion for -XUnliftedNewtypes in checkNewDataCon

Note [Implementation of UnliftedNewtypes], point says:

<Error Messages>: It's tempting to think that the expected kind for a newtype
constructor argument when -XUnliftedNewtypes is *not* enabled should just be Type.
But this leads to difficulty in suggesting to enable UnliftedNewtypes. Here is
an example:

  newtype A = MkA Int#

If we expect the argument to MkA to have kind Type, then we get a kind-mismatch
error. The problem is that there is no way to connect this mismatch error to
-XUnliftedNewtypes, and suggest enabling the extension. So, instead, we allow
the A to type-check, but then find the problem when doing validity checking (and
where we get make a suitable error message).

I think that Note refers to the code in GHC.Tc.TyCl.checkNewDataCon (which is transitively called from checkValidTyCon). There we have

        ; unlifted_newtypes <- xoptM LangExt.UnliftedNewtypes
        ; let allowedArgType =
                unlifted_newtypes || isLiftedType_maybe (scaledThing arg_ty1) == Just True
        ; checkTc allowedArgType $ vcat
          [ text "A newtype cannot have an unlifted argument type"
          , text "Perhaps you intended to use UnliftedNewtypes"
          ]

But that error emssage doesn't occur anywhere in the testsuite. If you try the example A from the Note, you instead get

test.hs:20:1: error:
    • Newtype has non-* return kind ‘TYPE 'IntRep’
      Perhaps you intended to use UnliftedNewtypes
    • In the newtype declaration for ‘A’
   |
20 | newtype A = MkA Int#
   | ^^^^^^^^^^^^^^^^^^^^

(Side note: It's a bit surprising that the emission of that error suppresses any following errors, for example of a following decl newtype B = MkB Int#. Perhaps this has to do with the next paragraph.)

That message (which I actually prefer) is emitted by GHC.Tc.Gen.HsType.checkDataKindSig.

It seems we can just get rid of the code in checkNewDataCon. Either way, we should update Note [Implementation of UnliftedNewtypes]. Maybe Split if off into its own Sub-Note? That Note is awfully long and has many many sub items.