Prune code related to reading agreement key from disk

**Background**

The "agreement key" is the mutual TLS key used in encrypting the network connection between nodes for transmitting gossip events. The agreement key uses as node's gossip signing key as a root certificate authority.

It used to be the case that we loaded the agreement key from disk. Since the gossip signing key is the root CA, however, there is no need to persist the agreement key, we can generate a new one at startup. We have since started to always generate a new agreement key when the node starts. The code base, however, still has logic for reading agreement keys from disk.

**Acceptance Criteria**

1. Remove any remaining logic that references loading agreement keys from disk.
* This includes any logic to delete agreement keys on disk.
2. Remove or modify the unit tests which have agreement keys on disk to no longer have agreement keys on disk.

Search the repository for logic that loads or deletes agreement keys from disk, then inspect the unit tests that create or expect on-disk agreement keys. Remove those remaining references and update the tests so agreement keys are generated at startup without disk state; completion is meeting both acceptance criteria with the unit tests passing.

Written by the indexing model from the issue text.