chore: Remove storage functionality from Delegation Controller

[jeffsmale90]

Explanation

Delegation "storage" was added the the Delegation Controller to support Remote Mode. Because this feature is no longer in development, the storage functionality is not used.

This PR removes entirely the storage functionality from the controller.

Background

#8286 aimed to migrate code from clients into the controller, but as this storage functionality is unused, it was decided to remove the functionality completely. Followup PRs will be opened in the clients to update the controller, and remove the remaining boilerplate, including awaitDeleteDelegationEntry function.

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

Medium Risk
This is a breaking API change: it removes delegation persistence state and all related messenger methods/types, so downstream consumers may fail at compile/runtime until updated. Runtime risk is otherwise limited since signing behavior remains and storage logic is deleted rather than rewritten.

Overview
DelegationController is now signing-only. The controller no longer persists or exposes any delegations state, and its state metadata/default state are reduced to an empty object.

All storage-related APIs are removed: store, list, retrieve, chain, and delete methods, their messenger method action types, and the DelegationEntry/DelegationFilter exports. This also drops the @metamask/accounts-controller dependency and updates tests/docs/changelog to reflect the breaking removal.

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