fix test for optional inputs of new plotSlice

Author: wahln

test/util/test_plotSlice.m

@@ -103,5 +103,4 @@
     assertTrue(isa(hIsoDose, "cell"));
     if ~moxunit_util_platform_is_octave
         assertTrue(isa(hDose, 'matlab.graphics.primitive.Image'));
-        assertTrue(isa(hCt, 'matlab.graphics.primitive.Image'))
     end