Rollup merge of #112053 - nnethercote:rm-Zcpu-partitioning-strategy, r=wesleywiser

Remove `-Zcgu-partitioning-strategy`.

This option was introduced three years ago, but it's never been meaningfully used, and `default` is the only acceptable value.

Also, I think the `Partition` trait presents an interface that is too closely tied to the existing strategy and would probably be wrong for other strategies. (My rule of thumb is to not make something generic until there are at least two instances of it, to avoid this kind of problem.)

Also, I don't think providing multiple partitioning strategies to the user is a good idea, because the compiler already has enough obscure knobs.

This commit removes the option, along with the `Partition` trait, and the `Partitioner` and `DefaultPartitioning` types. I left the existing code in `compiler/rustc_monomorphize/src/partitioning/default.rs`, though I could be persuaded that moving it into
`compiler/rustc_monomorphize/src/partitioning/mod.rs` is better.

r? ``@wesleywiser``

Author: matthiaskrgr