Remove support for MIPS

This discussion is prompted by https://github.com/python/cpython/issues/127708#issuecomment-2524172362

We do not release or test these distributions, but we build them.

We build the following targets for all Python verisons:

- `mips-unknown-linux-gnu`
- `mipsel-unknown-linux-gnu`

It also looks like there's support in the code for `mips64el-unknown-linux-gnuabi64` but we don't build them in CI.

I don't think these targets merit the compute and maintenance time. I propose just removing them from the project.

Please comment here if you're using these builds.