Allow removing HD accounts

From https://github.com/MetaMask/metamask-extension/issues/2028

I propose being able to delete seed phrase accounts as well (=accounts which were generated with "Create account").

For this, we have to decide what happens when someone creates an account after deleting one. The simple and safe option is that the previously deleted account becomes regenerated. It probably won't do what some people (including me) want it to do, but it's still better than not being able to delete organic accounts.

There could be another option for organic accounts, "archiving", which would remove them from the account list, but they would stay reachable in a segregated section.

I'm not sure about the added utility this would add compared to the UI complexity it introduces, especially for new users. Possibly a good candidate for a "dev mode" feature flag. Leaving here to gauge interest.

Removing loose accounts is already a feature in the new UI.

Start by reading the linked issue #2028 and the notes about removing loose accounts in the new UI. Clarify whether HD or seed-phrase accounts should be deleted or archived, how regeneration should work, and whether this belongs behind a developer-mode feature flag. Done requires an agreed behavior and corresponding account-management UI work.