Revert part of [9386d6f63468] because the new automatic function pointer binding...

Revert part of [9386d6f63468] because the new automatic function pointer binding cannot properly track per-context function mappings when the context is more complex than a single context-type pointer. e.g. it is fine for sqlite3_trace_v2() but it breaks down with sqlite3_create_collation() because that one needs to use the collation name as part of the context key and we cannot sensibly do so with the current code.

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