Halide should drop MIPS support

[steven-johnson]

The primary reason for adding this was for Android, but Android hasn't supported MIPS since NDK r17 (~2018), and even MIPS-the-company no longer supports this architecture (they have pivoted to RISC-V). We should prune it to simplify things.

[abadams]

Note also that our MIPS support is currently untested.

[steven-johnson]

It was always untested in the past, but it's currently untested, too