When the method del_lchan is called, closed_channel_nr still contains a dict
that contains the channel number under the key 'logical_channel_number'.
This will lead to an exception. We must extact the channel number from
the dict before we can use it with del_lchan. (See also
created_channel_nr)
Related: OS#6094
Change-Id: I399856bc227f17b66cdb4158a69a35d50ba222a7