Remove the ability to skip device verification during login

We will soon ignore devices not signed by their users when encrypting and decrypting room events.
As a consequence a non verified device will not be able to participate in an encrypted room (will not receive room keys, and room keys sent will be ignored).

Currently it is optional to verify your device on login, the flow can be skipped. We want it to be non-skippable now in preparation for the future change.