Remove darwinFlags in rust module

[tianyaochou]

Remove darwinFlags to fix linking error when compiling no_std crate. Closing #1355

Only tested with Developer Frameworks already installed in the system.

I am not sure if this change will break those who only rely on Developer Frameworks in Nixpkgs and don't have Developer Frameworks installed outside Nix.

[sandydoo]

I've updated our rust tests and examples to work with this change.

[domenkozar]

What if you need to compile against a framework library on macos?

[sandydoo]

@domenkozar, I tested this out in a fresh VM.
Nix already adds the -iframework CFLAG for us.
The -L framework rust flag doesn't seem necessary. I tested a bunch of random OSS projects that link to frameworks — they compile without issues. Is this around from the pre-1.0 era when we didn't have setup hooks?