Remove unused objc_exception dependency

[chyyran]

This one unused objc_exception dependency is preventing docs from building on docs.rs, because cc tries to pass -arch, to GCC, which doesn't see it as a valid argument, when building objc-exception.

Except, objc-exception doesn't really do anything in metal-rs. I grepped for all of the functions exported by objc-exception, and found nothing at all. I then ran

• cargo check --target=x86_64-apple-darwin
• cargo check --target=aarch64-apple-ios
• cargo check --target=aarch64-apple-darwin

On both Windows and Linux and it passed with flying colours. To be sure, I checked the source of the objc crate crate, where the exception feature only brings in objc-exception and this catch_exception function. There are no special linkage parameters that objc_exception provides nor is it used at all by metal-rs and thus wgpu.

There's no reason to keep this around especially because its the only reason docs are breaking for it. It's causing trouble for downstream consumers of libraries that use wgpu, I've had to resort to workarounds like #[cfg(all(target_vendor = "apple", feature = "docsrs"))] to remove wgpu dependencies on my macOS docs build chain.

This should fix gfx-rs/wgpu#2956, #5 , and obviate #250

I've also fixed a static_mut_ref warning that cargo was complaining about.

[madsmtm]

Yup, that's exactly right

[grovesNL]

Thank you!