ci: Remove MIPS* from CI

MIPS was recently downgraded to Tier 3, which means it won't be installable by
rustup and may not work as expected. This commit removes all MIPS-related
CI jobs.

.github/workflows/cross.yml

@@ -27,10 +27,6 @@ jobs:
           - arm-unknown-linux-gnueabi
           - arm-unknown-linux-gnueabihf
           - armv7-unknown-linux-gnueabihf
-          - mips-unknown-linux-gnu
-          - mips64-unknown-linux-gnuabi64
-          - mips64el-unknown-linux-gnuabi64
-          - mipsel-unknown-linux-gnu
           - powerpc-unknown-linux-gnu
           # - powerpc64-unknown-linux-gnu  # not supported by cross
           - powerpc64le-unknown-linux-gnu