Remove LICENSE.GPLv3, LICENSE.LGPLv21, LGPL_EXCEPTION.txt

Commit 71404b0 added new header templates and LICENSE.GPL2,
LICENSE.GPL3, LICENSE.GPL3-EXCEPT, LICENSE.LGPL3 files already in 2016.
So it's finally time to remove some of the old licnse files.

LICENSE.LGPLv21 and LGPL_EXCEPTION.txt is not referenced anywhere
anymore. LICENSE.GPLv3 is referenced only in LICENSE.LGPLv3.
We unfortunately still have to keep LICENSE.LPGLv3 because it is up
to this day used in the standard header templates, but we can just
change the reference in there to LICENSE.GPL3.

Change-Id: Ia88b4de8a395757a39ca63d474b2214e3b1063fb
Reviewed-by: Jani Heikkinen <[email protected]>