libs/agg: Remove GPC.

It was licensed with a "no commercial usage allowed" license, which if
we were using it would be a gray area at best. We weren't using it at all,
and AGG seems to have disabled building it by default, so just remove it
from the tree entirely.

Briefly discussed on IRC.

data/system/data/licenses/GPC

@@ -1,21 +0,0 @@
-           http://www.cs.man.ac.uk/aig/staff/alan/software/
-
-Author:    Alan Murta (email: [email protected])
-Version:   2.31
-Date:      4th June 1999
-
-Copyright: (C) 1997-1999, Advanced Interfaces Group,
-           University of Manchester.
-
-           This software is free for non-commercial use. It may be copied,
-           modified, and redistributed provided that this copyright notice
-           is preserved on all copies. The intellectual property rights of
-           the algorithms used reside with the University of Manchester
-           Advanced Interfaces Group.
-
-           You may not use this software, in whole or in part, in support
-           of any commercial product without the express consent of the
-           author.
-
-           There is no warranty or other guarantee of fitness of this
-           software for any purpose. It is provided solely "as is".