Fix method checks to also look into superclasses if method was overridden

[uschindler]

While testing Java 9, I found the following bug:

The tests for AccessibleObject#setAccessible() were not finding violations on Java 9 anymore. The reason is that Jigsaw overrides AccessibleObject#setAccessible() in the Executable subclass. Because of that the signature was already seen before and forbiddenapis did not look into superclasses. So the violation was not found (1 test failed).

This is a general bug of forbiddenapis, the old checks were easily able to break if a forbidden method was overridden by a subclass!

This fix removes the check that prevented looking into superclasses. Fields are not affected, because fields are never "virtual", so the "optimization" can stay.