chore(login): Remove the legacy `contentToken` parameter from login
chore(login): Remove the legacy `contentToken` parameter from login

https://github.com/mozilla/fxa-auth-server/pull/1580

r=philbooth
mozilla/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server