Bug 1038676 — Disable MOMT flow if no free number available.

https://bugzilla.mozilla.org/show_bug.cgi?id=1038676
I think we shouldn't have this condition. The idea is to have either verificationMethods = ["sms/momt", "sms/mt"] or ["sms/mt"].
If we don't know the msisdn when we call discover and there is no moVerifier, we will fail with "Invalid MSISDN number" while we should just send verificationMethods = ["sms/mt"]. The client will make sure that we ask the user for the phone number. This may change in the future and the client may fallback to MT if no verification methods are provided, but for fxos 2.0 we shouldn't make any more changes in the client if possible.