refactor: partially invalid exception thrown

[LiuXing-R]

1, Remove useless PolicyException
2, The PolicyException thrown is meaningful after the custom policy parameter is judged null

[davewichers]

@spassarop - what do you think of this proposed change?

[spassarop]

Changes make sense. The throws in constructors were already useless, the null policy exception moved a level below is still valid and maybe cleaner (avoids repeating the throw).

The one change I'm not sure is the main method on AntiSamyDOMScanner and not because it's useful, it's clearly not. But for if there is a real reason for an empty main on the project. @davewichers, is there any reason for en empty main to exist? Something about compulsary executable class for the Maven pipeline or whatever? If it can be deleted then the PR is all right.

[LiuXing-R]

Two useless throws were missed.

[LiuXing-R]

@davewichers
I've found that Antisamy has a lot of redundant code, and I'd be happy to refactor it if you agree.

[davewichers]

@spassarop - I doubt the main() is needed. @LiuXing-R - I'm going to accept this pull request. If you want to work on refactoring to clean up the code, feel free. Thanks!