docs: revert threat model for node (#24465) (#24610)

Removes `yarn-project/THREAT_MODEL.md`, added in
[#24465](#24465).

Per discussion in #team-alpha: detailed threat models and invariants may
make it easier for attackers to find bugs, so the team wants this
content pulled from the public repo for now. It will be re-added to
LabsBox/ClaudeBox (private/internal) — see
[claudebox#1357](AztecProtocol/claudebox#1357) —
and can come back here once the team is finding fewer bugs.

Only the threat model doc itself is removed here — the incidental doc
cleanups bundled into the same squashed commit (stale BLOCK protocol
references in `yarn-project/p2p/README.md` and
`yarn-project/p2p/src/services/reqresp/README.md`, and the missing
`DUPLICATE_ATTESTATION` entry in `yarn-project/slasher/README.md`) are
kept as-is.

(cherry picked from commit 4d02692)

Author: AztecBot