'Remove a local or remote user from a local user\'s presence list and
unsubscribe the local user from updates that user.
Args:
observed_user(UserId): The local or remote user to remove from the
list.
observer_user(UserId): The local owner of the presence list.
Returns:
A Deferred.'

@defer.inlineCallbacks
def drop(self, observed_user, observer_user):

    if (not self.is_mine(observer_user)):
        raise SynapseError(400, 'User    is    not    hosted    on    this    Home    Server')
    (yield self.store.del_presence_list(observer_user.localpart, observed_user.to_string()))
