salsa20-core: warnings for word_idx

[tarcieri]

cc @emc2 @sseemayer

There's presently a warning for salsa20-core:

https://github.com/RustCrypto/stream-ciphers/blob/1dc6af/salsa20-core/src/salsa_family_state.rs#L96

Warning from CI:

https://travis-ci.org/RustCrypto/stream-ciphers/jobs/571061677#L202

warning: value assigned to `word_idx` is never read
  --> salsa20-core/src/salsa_family_state.rs:96:25
   |
96 |                         word_idx = 0;
   |                         ^^^^^^^^
   |
   = note: #[warn(unused_assignments)] on by default
   = help: maybe it is overwritten before being read?

It's unclear to me what the intention was here, or if it can simply be removed. The "maybe it is overwritten before being read" help text has me curious if in the refactor it isn't being preserved somewhere it needed to be read, or if it's now being unconditionally overwritten in a way it shouldn't be.