Auto merge of #119265 - Mark-Simulacrum:remove-cache, r=cjgillot

Remove metadata decoding DefPathHash cache

My expectation is that this cache is largely useless. Decoding a DefPathHash from metadata is essentially a pair of memory loads - there's no heavyweight processing involved. Caching it behind a HashMap just adds extra cost and incurs hashing overheads for the indices.

Based on rust-lang/rust#119238.

Author: bors