Prevent warning on unused ret

It is used in asserts for debugging only anyway.
Let app exit when wire_io is inactive

libwire will exit when there is no action to take, but wire_io kept the
wire_fd active all the time by way of the communication channel to the
threads. By making wire_io disable this waiting when there are no
IO requests to handle it lets applications exit without special
handling.

This makes wire_io_shutdown superfluous.
Remove wire_io_shutdown