SCIFIOCellImgTest: remove out-of-scope test

The test for ImgPlus disposal after garbage collection is failing
repeatedly for unknown reasons on Jenkins. It is not strictly necessary
as we have other tests for the RefManagerService, and there is still a
test for the disposal mechanism. It would be great to get this working -
but requires more understanding of how to reliably test ref queue
operations after GC (and perhaps it's just not possible..).

Note that this test DOES pass in many local environments, and has even
passed on Jenkins previously. So certainly the behavior is capable of
working.