Rollup merge of #148306 - zetanumbers:expn_id_decode, r=nnethercote

Remove double check when decoding ExpnId to avoid races

Fixes debug assertion failure as described in rust-lang/rust#141540 (comment)

Essentially failure happens during the race while decoding one `ExpnId` from different threads. This ICE doesn't happen with single threaded thread_pool due to early return within `decode_expn_id` with the same condition:

https://github.com/rust-lang/rust/blob/8205e6b75ec656305ac235d4726d2c7a1ddcef14/compiler/rustc_middle/src/query/on_disk_cache.rs#L605-L607

However I believe this race does not hurt because `register_expn_id` is pretty much idempotent:

https://github.com/rust-lang/rust/blob/8205e6b75ec656305ac235d4726d2c7a1ddcef14/compiler/rustc_span/src/hygiene.rs#L1397-L1413

Author: GuillaumeGomez