Remove MIPS from CI

[Amanieu]

• MIPS targets have been removed from rustup, see Drop MIPS to tier 3 compiler-team#648.
• Removed #[target_feature] requirements from const fn, since that caused const-eval to fail.

[rustbot]

@Amanieu: no appropriate reviewer found, use r? to override

[Amanieu]

@alexcrichton It seems that there has been some breakage to WASM in stdarch. Specifically, the problem is that functions like i8x16 are defined as const fn but also have a #[target_feature(enable = "simd128")], which now causes them to fail in const-eval. I've removed the target feature attribute from all const fn, since those should never require the target feature anyways.

[alexcrichton]

Apologies for the delayed response but that sounds good to me! Given that these are all constant-creating things they don't need the feature anyway to work with LLVM's codegen so all seems reasonable to me. Thanks for taking care of it!