jamiaccount: remove connections and data on banned

A contact can be banned from another device. In this case,
JamiAccount::removeContact is not called and the daemon only call the
dedicated callback.
So, remove the conversation requests and connections to the banned
contact (and avoid future incoming calls if connected) when the
callback is called instead in JamiAccount::removeContact() as it
will be called in a multi-device context.

Change-Id: I1ac8b441268d7bfc9bb6627fbbbc7b83e2e75052