Remove winGRASS 6 from OSGeo4W

[hellik]

Following the discussion in

​https://lists.osgeo.org/pipermail/grass-dev/2018-September/089629.html

There is now an effort to bring GRASS to Python 3.

GRASS 6 is nowadays outdated and not maintained anymore, and is ready to be archived.

To reduce too many and different winGRASS related packages (and dependencies) in OSGeo4W, GRASS 6 should be removed from OSGeo4W.

This would also ease the maintainance and helps cleaning during transition to Python 3