Remove invalid clause about generated code from licence file

The licence statement (COPYING.txt) apparently contained a sentence stating that all generated kernels would be subject to the AGPLv3. This is nonsensical and legally void. This is clarified in the GPL FAQ.

COPYING.txt

-------------------------   Important ---------------------------------
-
-lbmpy is under the following GNU AGPLv3 license. 
-This license holds for the sources of lbmpy itself as well 
-as for all kernels generated with lbmpy i.e. 
-the output of lbmpy is also protected by the GNU AGPLv3 license. 
-
-----------------------------------------------------------------------
-
-
-
-                    
                     GNU AFFERO GENERAL PUBLIC LICENSE
                        Version 3, 19 November 2007