Logical error in flavors unset_keys method

When we want to unset multi keys of flavor
extra data, only first key can be removed
because of the key word named 'return'.
To fix it, remove the return key word

Change-Id: I70d1f2e84a2bfcea1db60b26138f84421f9a7309
Closes-bug: #1324077
Merge "Logical error in flavors unset_keys method"