useLedgerLive setting isn't always in sync

[darkwing]

This is fairly simple to reproduce:

1. Connect a Ledger account to MetaMask
2. (eth-ledger-bridge-keyring injects the github pages iframe with correct useLedgerLive setting)
3. Use the test dapp to perform a signature
4. Remove the Ledger account from MetaMask
5. Add the account again

The useLedgerLive setting will be false in the IFrame, leading to the user's browser hanging (while the iframe incorrectly tries U2F.

Presently MetaMask sends the useLedgerLive setting down to the iframe when the user logs in, and when they toggle the setting. That isn't reliable enough in some cases.

[darkwing]

The best solution here is probably going to be removing the useLedgerLive setting everywhere (extension and keyring) and simply enforcing Ledger Live for Chrome (since U2F won't be returning to Chrome) and U2F for Firefox.