salsa20: revert sse2 (#346)

This reverts #328.

The changes introduced here generate failure when used in `scrypt`. The
`scrypt_block_mix` would generate a different value.

Note that this change was never released, so there is no security impact.

Author: baloo

Cargo.lock

@@ -13,7 +13,6 @@ keywords = ["crypto", "stream-cipher", "trait", "xsalsa20"]
 categories = ["cryptography", "no-std"]
 
 [dependencies]
-cfg-if = "1"
 cipher = "=0.5.0-pre.4"
 
 [dev-dependencies]