refactor(multichain-account-service): remove old unused event

[ccharly]

Explanation

I forgot to remove this event in my previous PR:

• feat!: move SnapPlatformWatcher in SnapAccountService + add :ensureReady and use it in MultichainAccountService #8715

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk cleanup that only tightens messenger event typings/delegations and updates related lint suppression/changelog entries; behavioral impact should be limited to consumers that still attempted to delegate/subscribe to KeyringController:stateChange.

Overview
Removes the unused KeyringController:stateChange subscription from multichain-account-service by dropping it from the test messenger delegation list and from the service’s AllowedEvents type.

Updates supporting metadata: deletes the now-unneeded ESLint suppression for src/tests/messenger.ts and tweaks the changelog entry to reflect that KeyringController:stateChange is no longer required (and references the follow-up PR).

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