Remove `connection_info.peer_used_cert_chain_slot_id`

[steven-bellock]

From the vantage point of #3423 and #3435, it looks like connection_info.peer_used_cert_chain_slot_id can be removed after those pull requests have been merged. In particular, aside from the KEY_EXCHANGE flow, storing the slot_id value in the context evaded passing it as a function parameter to functions such as libspdm_get_peer_cert_chain_buffer. However, now that those functions have a proper slot_id parameter (once the two pull requests are merged), connection_info.peer_used_cert_chain_slot_id can be removed.