Removes mips toolchains from the rust CI build

The error we were getting was:
    error: component 'rust-std' for target 'mips*' is unavailable for download for channel '*'

I have not investigated further. These toolchains are 'Tier 3' here:
     https://rust-lang.github.io/rustup-components-history/
which is a likely factor for them being less supported.

Author: LiquidityC

.github/workflows/rust.yml

@@ -31,10 +31,6 @@ jobs:
           i586-unknown-linux-gnu,
           i686-unknown-linux-gnu,
           i686-unknown-linux-musl,
-          mips-unknown-linux-gnu,
-          mips64-unknown-linux-gnuabi64,
-          mips64el-unknown-linux-gnuabi64,
-          mipsel-unknown-linux-gnu,
           powerpc-unknown-linux-gnu,
           powerpc64-unknown-linux-gnu,
           powerpc64le-unknown-linux-gnu,