Drop support for PNaCL

[pitdicker]

Now that rustc and Google now longer support it, it seems we can remove the code from OsRng.
See rust-lang/rust#45041 and rust-lang/rust#42420

[dhardy]

Be nice to remove that, but issue linked (#42420) mentions that only PNaCl is deprecated, not Google NaCl.

[pitdicker]

Hmmm, yes. It is a bit messy

From rust-lang/rust#42420 (comment):

I think NaCl has gotten some use outside the browser, so I could imagine a case for still supporting NaCl in Rust. It's probably only worth doing if there are current users of it though.

I read that as: the primary use case for target_os = "nacl" is to support running Rust code in browsers. That is not available anymore. Maybe there is someone be uses Rust together with the NaCl sandboxing technology. Probably not, but we don't know.

Personally, I would rather remove it. If there is really somebody using it, say sorry and add it back.
Instead of modifying it now to add proper error handling, and going through the trouble to somehow test it.