Remove unnecessary cleanup procedures of WmTests

Since WMS instance is created for each test methods, the cleanup for
the window created by previous tests is not needed.

Take ZOrderingTests as the example, it reduces about ~18% (20s+).
If there are more windows to remove, the improvement will be more
significant.

Bug: 154655192
Test: presubmit
Change-Id: If2cfdd15688fd72d37bab7caf6ab1f27766ba7f6