s3-printing: remove printer_list_need_refresh

s3-printing: remove printer_list_need_refresh

printer_list_need_refresh() was used previously to ensure one smbd
process did not attempt to update the printer_list tdb during or soon
after update by another smbd.

It is no longer needed, as pcap updates are now only performed by the
parent smbd process following startup, SIGHUP, config update or printcap
cache time expiry.