remove redundant calculations
Merge branch 'aql2' of https://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/ExecutionNode.cpp
	arangod/Aql/ExecutionNode.h
	arangod/Aql/ExecutionPlan.cpp
	arangod/Aql/OptimizerRules.cpp