Remove unrequired callled to getVoldriverReturns
Remove unrequired callled to getVoldriverReturns

[#149934726](https://www.pivotaltracker.com/story/show/149934726)

Signed-off-by: Kai Xiang <987005552a526059a0c9b4b70d166beabef8374e@pivotal.io>

cloudfoundry-incubator/volman,cloudfoundry-incubator/volman