remove stablemap from hhvm tests

Summary: This diff deletes usages of stablemap from tests where we
also do the same thing with a map, if not, it replaces stablemap with map.
Also deleted a test only for stablemap

Reviewed By: @paroski

Differential Revision: D1653680

Signature: t1:1653680:1414799769:9748e9254ea2cdae0e0f5eecdd337e89b65bee1a