Fix: forget account for legacy account without authorizedAccounts

[Tbaut]

closes #1376

I managed to reproduce, here are the details for this to happen:

• you need a state with the old version of the extension where you have a non empty authUrls list, which means that you have ever connected at least an account to a dapp. Checkout commit 4edd9cd for instance.
• once this is done, upgrade the extension to master: there's no state migration per se
• if you ever visit a website you had visited before, one that is present in your authUrls, then you'll need to set the authorizations for this website again, as in, it used to be available for all accounts, now you need to specify. Doing so will set authorizedAccounts for this website.
• if you try to forget an account, and you have at least one entry in your authorizedAccounts that doesn't have authUrls, then it'll crash.

The fix is super simple once this is clear, authorizedAccounts can be undefined and shouldn't prevent us from removing an account.

[TarikGul]

Thank you so much! Awesome reproduction!

[polkadot-js-bot]

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