[datafeed] do NOT call commit in DataFeedXMLParser.process() (closes #10928592)
and so, remove more useless code.
Doing a commit in DataFeedXMLParser.process() is a bad idea because this method
is called for every imported/updated entity, before handling deletions. So if
we commit here, odds are quite high that we're violating some integrity
constraint and end up with a ValidationError.