Remove unused dep rust-crypto from hphp
Summary:
The unmaintained rust package `rust-crypto` is recommended for
deprecation; it turns out we weren't using it in `hphp/src/decl` but
we were declaring it as a dependency.
To update Cargo.toml and dependencies, I ran
```
buck run //common/rust/cargo_from_buck:cargo_from_buck
```
and updated the vendor info
```
hphp/hack/scripts/facebook/cargo_vendor.sh && hphp/hack/scripts/facebook/cargo_vendor.sh
```
Reviewed By: jsgf
Differential Revision:
D20589163
fbshipit-source-id:
af1eb757ced28f4b5cb22430e5d97ef79883de49