crypto: arm64/ghash - drop PMULL based shash

There are two ways to implement SIMD accelerated GCM on arm64:
- using the PMULL instructions for carryless 64x64->128 multiplication,
  in which case the architecture guarantees that the AES instructions are
  available as well, and so we can use the AEAD implementation that combines
  both,
- using the PMULL instructions for carryless 8x8->16 bit multiplication,
  which is implemented as a shash, and can be combined with any ctr(aes)
  implementation by the generic GCM AEAD template driver.

So let's drop the 64x64->128 shash driver, which is never needed for GCM,
and not suitable for use anywhere else.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>