Remove sync-pull-masters step

The sync-pull-masters step runs "scap pull" on all deploy servers other than the current. This step is a holdover from a time when deploy servers needed to have a populated /srv/mediawiki directory in order to properly act as deploy servers. This hasn't been the case for a while so remove this unnecessary operation. Deploy servers that are marked as mediawiki installation targets will still run "scap pull" during the sync-apaches phase.