Removed register_attr from required rust features when compiling for spirv

[oisyn]

Hi.

I took the liberty of removing the register_attr feature since it's no longer supported on Rust nightly since September 2022. It doesn't seem that the feature is actively being used anywhere in the glam codebase.

I'm working on rust-gpu, which requires the Rust nightly toolchain and am in the process to update to a more recent nightly toolchain. Since one of our crates, spirv-std, is optionally dependent on glam, I'm getting compile errors because of the feature(register_attr).