VirtualTableViewerTest suppresses a warning that will no longer be raised

[stephan-herrmann]

Once we merge eclipse-jdt/eclipse.jdt.core#5054 the method org.eclipse.jface.tests.viewers.VirtualTableViewerTest.testSetAllSelection() will raise a new warning:

Unnecessary @SuppressWarnings("unchecked")

At that point the suppression should simply be removed.