REV: Revert adding a default ufunc promoter

Adding a default promoter should not be necessary with the
reduction-hack (use output DType to guess loop DType if incompatible
with the loop).

However, using it had two effects:
1. SciPy has a ufunc that has a loop it does not want to exist.
   This loop is homogeneous (unlike the correct one) and gives
   a deprecation warning.  The default promoter would assume the
   homogeneous loop is OK (maybe not ideal) if it exists.
   Since it is a "bad" loop that gives a deprecation warning, it
   is not really true though.
2. Datetime promotion is currently utterly buggy, leading to:
       timedelta.sum(dtype="f8")
   ignoring the `f8` requests.  But we actually end up relying on
   that behaviour in our `np.mean` implementation...

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