Stop automagically select default encryption

Selecting a default encryption (in our case OMEMO) has several down sides.
First of all users might have perfectly valid reasons not to use encryption
at all such as using the same private server. Second of all the way it was
implemented Conversations would automatically fall back to plain text as soon
as the conditions changed (recipient switches to device with no encryption)
which lead to unexpected situations.
Thirdly having a default encryptions speaks against the 'mission
statement' of Conversations of not forcing its security and privacey
aspects upon the user.
And last but not least the goal of implementing this feature in the
first place: Be encrypted by default didn't work at all. I don't think
there was a single user that we succesfully 'tricked' into using OMEMO
who otherwise wouldn't have used it.