Remove `powerpc64le-unknown-linux-musl` target (#9810)
## Summary
This is blocking the release (#9793). We seem to have hit some sort of
limit that's causing builds to fail on this target. It's a Tier 3 Rust
target with _unknown_ (???) `std` support (see the question mark
[here](https://doc.rust-lang.org/rustc/platform-support.html)).
