Remove conversion from Hash to Hashing

[ctongfei]

scala.util.hashing.Hashing[A] presupposes universal equality using == -- a == b implies hash(a) == hash(b).
cats.kernel.Hash[A]'s underlying eqv is based upon cats.kernel.Eq which is customizable -- eqv(a, b) implies hash(a) == hash(b).
One cannot convert a cats.kernel.Hash[A] to scala.util.hashing.Hashing[A] without compromising the typeclass instance's semantics.
@kailuowang #2116

[kailuowang]

@ctongfei thanks for point it out. We had a discussion with @denisrosset in #2116, according the scala API doc

Note: when using a custom Hashing, make sure to use it with the Equiv such that if any two objects are equal, then their hash codes must be equal.

So it seems that we should be able to convert Hash to Hashing and Eq to Eqiv?

[ctongfei]

Thanks @kailuowang for pointing out that. I agree with your justification.