Remove Yahoo from email providers that can request bridges

Since June 2019, BridgeDB only works with Riseup and Gmail.

From now on, users can no longer request bridges from a Yahoo email account, which fixes issue #28496. We believe that Yahoo fell behind in making it hard for spammers to create many email accounts, and it also has a feature that allows the creation of up to 500 disposable email addresses, which makes it easy for a censor to request a disproportionately large number of bridges. We therefore deactivated Yahoo, which leaves us with Gmail and Riseup as the email providers from which users can request bridges. https://blog.torproject.org/new-release-bridgedb-071