MINOR: protocol: do not call proto->unbind_all() anymore

MINOR: protocol: do not call proto->unbind_all() anymore

Similarly to previous commit about ->bind_all(), we have the same
construct for ->unbind_all() which ought not to be used either. Let's
make protocol_unbind_all() iterate over all listeners and directly
call unbind_listener() instead.

It's worth noting that for uxst there was originally a specific
->unbind_all() function but the simplifications that came over the
years have resulted in a locally reimplemented version of the same
function: the test on (state > LI_ASSIGNED) there is equivalent to
the one on (state >= LI_PAUSED) that is used in do_unbind_listener(),
and it seems these have been equivalent since at least commit dabf2e264
("[MAJOR] added a new state to listeners")) (1.3.14).