Removed incorrect comment about internal WGL/ARB definitions.

I actually had put those comments in because I had meant to go back and
re-check the WGL extension. But, maybe locally defined extensions
should be kept in a separate include file?

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@713 7f0cb862-5218-0410-a997-914c9d46530a