Cleanup license files

I was just grep-ing for GPL and found
./platform/o.n.core/licenses/LICENSE.txt which mentions that

> The Common Development and Distribution License
> (CDDL), Version 1.0 governs your use of:
> NetBeans Platform software

There's also ./platform/o.n.core/licenses/THIRDPARTYLICENSEREADME.txt
with the text of GPL w/ CPE and it seems to be related to Swing Layout
Extensions which I don't believe we include anymore (or do we?).

And finally there's  ./platform/o.n.core/licenses/DISTRIBUTION.txt for
JavaHelp 2.0 which we also excluded under Apache.

The whole ./platform/o.n.core/licenses/ folder should be removed.

There's also ./nbbuild/standard-nbm-license.txt with the CDDL and GPL
w/ CPE text which probably should also be deleted.