Remove "OR a.parent_id=0"

Remove "OR a.parent_id=0" which was accidentally reintroduced with a platform merge (https://github.com/joomla/joomla-cms/commit/8f0beaf4195338519884215b1405b843db4a914f).

This created a performance problem due to caching issues.
Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31497