GH-15994: fix suggestion that anonymous classes be made abstract (GH-15995)

In the process, remove the (incorrect) assumption that any abstract method that
needs to be implemented by a c

GH-15994: fix suggestion that anonymous classes be made abstract (GH-15995)

In the process, remove the (incorrect) assumption that any abstract method that
needs to be implemented by a class that cannot itself be made abstract must be
a private method - the existing test for an enum already showed that this was
not the case.

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