Don't try to read globals from PurgePrivateVoteData::__construct()

Globals aren't loaded for Maintenance subclasses until well after the
instance is constructed.

I left the local variable in there in case someone wants to add a
command line option for it at some point (or if some magic is already
making one).

Change-Id: Ib0213e8f5e3f5ac866f42140e3c8be05fa39d753