kiji-express: EXP-398. Cleanup type parameter code in KeyValueStore.scala.

issue: https://jira.kiji.org/browse/EXP-398
review: https://review.kiji.org/r/1551
commit: https://github.com/kijiproject/kiji-express/commit/6d7ee0f1b8f295e06e607d739eae7218e1c14d29