handle remove_line for sccp

problem was: when removing a sccp line from a user, the sccp device
was still seen as configured, but no line was associated to it. This
situation would make the device invisible from the webi, because
entity filter makes it indistinguisable from devices beloging to other
entities:

1. a configured device with no line really has no line,
2. a configured device with a line in another entity still has no
line, because of the entity filter applying on the list of lines
3. a configured device with a line in the same entity has a line
Merge branch '4393-remove-line-from-sccp'