Note [Remove redundant provided dicts]

Recall that
   HRefl :: forall k1 k2 (a1:k1) (a2:k2). (k1 ~ k2, a1 ~ a2)
                                       => a1 :~~: a2
(NB: technically the (k1~k2) existential dictionary is not necessary,
but it's there at the moment.)

Now consider (#14394):
   pattern Foo = HRefl
in a non-poly-kinded module.  We don't want to get
    pattern Foo :: () => (* ~ *, b ~ a) => a :~~: b
with that redundant (* ~ *).  We'd like to remove it; hence the call to
mkMinimalWithSCs.

Similarly consider
  data S a where { MkS :: Ord a => a -> S a }
  pattern Bam x y <- (MkS (x::a), MkS (y::a)))

The pattern (Bam x y) binds two (Ord a) dictionaries, but we only
need one.  Again mkMimimalWithSCs removes the redundant one.