Remove sha2 dep once bitcoin_hashes provide hardware accelerated sha256

[RCasatta]

rust-bitcoin/rust-bitcoin#1962
rust-bitcoin/rust-bitcoin#1424

[RCasatta]

@romanz: released rust bitcoin now has hardware accelerated sha on x86 platforms, I would like to drop sha2 feature/dep in its favor but I think we will be missing acceleration on arm, should we do it or wait for bitcoin_hashes to add hardware acceleration on arm?

[romanz]

Good point, thanks!
It seems indeed that sha2 has specific optimizations for ARM-based devices:
RustCrypto/hashes#490
RustCrypto/hashes#507
https://github.com/RustCrypto/hashes/tree/master/sha2/src/sha256
If possible, let's wait :)