WorkWrap: Deprecate `-ffun-to-thunk` in favor of full laziness

In #21192 (closed), I dicovered that -ffun-to-thunk is basically useless, because full laziness will float out a thunk anyway. E.g., I tried the following program:

f :: Int -> Int
f x = sum (g x) + sum [0..x] + sum (g (x+2))
  where
    g _ = [0..x]
    {-# NOINLINE g #-}

and even without -ffun-to-thunk, we get

$wf
  = \ ww_s1In ->
      let { lvl_s1J6 = eftInt 0# ww_s1In } in
      let { $wg_s1HW = \ @ p_s1HU (_ :: Void#) -> lvl_s1J6 } in
      ...

So even though we don't make $wg into a thunk, we still float out a thunk out of $wg's constant body. So it appears -ffun-to-thunk doesn't have any effect after all if -ffull-laziness is on. To put it another way:

ftt	full laziness	do we get a thunk?
on	on	yes
off	on	yes
on	off	yes
off	off	no

As you can see, -ffun-to-thunk only makes a difference if full laziness is off. Who deactivates full laziness but wants WW to still create thunks by passing -ffun-to-thunk?!

Therefore, I propose that -ffun-to-thunk is always off and the flag deprecated.

The advantages are:

1. One flag less (to explain)
2. -ffull-laziness governs whether or not we will float out a thunk
3. We get to delete the code below

Code below:

      -- If fun_to_thunk is False we always keep at least one value
      --   argument: see Note [Protecting the last value argument]
      -- If it is True, we only need to keep a value argument if
      --   the result type is (or might be) unlifted, in which case
      --   dropping the last arg would mean we wrongly used call-by-value
      needs_a_value_lambda
        = not fun_to_thunk
          || might_be_unlifted

      -- Might the result be lifted?
      --     False => definitely lifted
      --     True  => might be unlifted
      -- We may encounter a representation-polymorphic result, in which case we
      -- conservatively assume that we have laziness that needs
      -- preservation. See #15186.
      might_be_unlifted = case isLiftedType_maybe res_ty of
                            Just lifted -> not lifted
                            Nothing     -> True

Since not fun_to_thunk and thus needs_a_value_lambda will always be true, we can delete all this.