Remove redundant preferences part 1

This is the first part of the preference cleanup... there are still a few more left to remove, but they are not trivial so I will do them later as a separate pull request.

I have left the `BroadcastMessages` class intact, while deleting the preference. I did this just in case we want to use it in the future to send a message to users, for example if there were to be a really critical bug in an alpha (i.e. distributed outside Google Play channel), and we needed to inform users to upgrade ASAP. I think it's safe to assume that if we do send a message on this channel, that it will be so important that we don't users to be able to disable it, hence the preference is not necessary.