Remove the TestBlockOpacity test

TestBlockOpacity is using vtkCompositePolyDataMapper2 that was deprecated
for 9.3.
Remove this test here as it won't pass after the fix introduced in
79969286.
Backporting 79969286 to fix the TestBlockOpacity is easy, but there is no
point in fixing the deprecated class. The functionality is still tested
with vtkCompositePolyDataMapper in Rendering/Core.