Drop clock_gettime implementation in favor of upstream

[ian-h-chamberlain]

It looks like libctru has implemented the clock_gettime syscall now: devkitPro/libctru@813d28d

As much as I prefer to have things implemented in Rust, it's probably best if we defer stuff like this to the canonical implementation upstream and reduce the surface area of this crate. We'll have to wait for a new release of libctru (which means we probably also should document some minimum required version of libctru somewhere...)

[Meziu]

I knew about this once the PR was opened, but I don’t see a new release available, has it released yet?

[ian-h-chamberlain]

No, it hasn't been released yet, so I think we can just leave this open to track it until a new release happens.

[Meziu]

Closed with #15