crypto/internal/fips140/sha3/_asm: remove unnecessary x/crypto dependency

crypto/internal/fips140/sha3/_asm: remove unnecessary x/crypto dependency

There is no need to blank-import golang.org/x/crypto/sha3, as we are not
using any crypto.SHA3 variant in the code.

Change-Id: Ia5455647f737371fc4ec0972bf9a90d5ee854495
Reviewed-on: https://go-review.googlesource.com/c/go/+/637055
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>