Remove nodes_in_current_session field and related assertions

[Zoxc]

This removes the nodes_in_current_session field and related assertions. These are enabled if -Z incremental-verify-ich is passed or debug_assertions is on. Historically these have been useful to catch query keys with improper HashStable impls which lead to collisions.

We currently also check for duplicate nodes when loading the dep graph. This check is more complete as it covers the entire dep graph and is enabled by default. It doesn't provide a query key for a collision however. This check is also delayed to the next incremental session.

We also have the verify_query_key_hashes which is also enabled if -Z incremental-verify-ich is passed or debug_assertions is on. This checks for dep node conflicts in each query cache and provides 2 conflicting keys if present.

I think these remaining checks are sufficient and so we can remove nodes_in_current_session.

[rustbot]

r? @jdonszelmann

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[Zoxc]

cc @nnethercote @Zalathar @zetanumbers