Remove `nodes_in_current_session` field and related assertions
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`.
Jonathan Brouwer
8bade0362d