SwContentTree::pActiveShell is not cleared because of a check that the
shell exists, which is only done in the special case bIsConstant.

* at most one of bIsActive or bIsConstant is true
* the only place that calls SetActiveShell() if bIsConstant is true
  already checks this condition

So remove the unnecessary check from SetActiveShell()

Change-Id: Id3b3b98bd38303b5afc529cc8be669872bd354b7