Get rid of CFMetaData isPurged(), markPurged()

CASSANDRA-3578 added a new isPurged field to CFMetaData. It's only being used by CommitLogSegment.markDirty().

The linked patch removed the field and all the purge-related methods, replacing that condition with !Schema.instance.hasCF().

That said, I'm not sure that the check makes sense at all, with CASSANDRA-5202.