Remove `HybridBitSet` `HybridBitSet` was introduced under the name `HybridIdxSetBuf` way back in #53383 where it was a big win for NLL borrow checker performance. In #93984 the more flexible `ChunkedBitSet` was added. Uses of `HybridBitSet` have gradually disappeared (e.g. #116152) and there are now few enough that they can be replaced with `BitSet` or `ChunkedBitSet`, and `HybridBitSet` can be removed, cutting more than 700 lines of code. r? `@Mark-Simulacrum`