WorkSource: Fix WorkSource#remove for chained worksources.

- It doesn't make sense to clear the list of WorkChains if we're
  remove is called with a WorkSource that has no chains.
- The early return for mNum <= 0 is faulty because it incorrectly
  returns early for WorkSources that have workChains but no flat
  UIDs.

Test: WorkSourceTest
Bug: 62390666
Change-Id: I4c0e69bdd7c114b41329aa329d1c1d08a8cb9b59