patches: Remove arrow from exit sign.

Having an arrow on the exit sign makes some levels confusing, as the
arrow can sometimes be pointing in the wrong direction or be
otherwise misleading. There is some discussion on Doomworld:
http://www.doomworld.com/vb/post/1303596

As the arrow version still looks quite nice, keep it around as a
second exit sign texture (EXITSGN2); include both the red and the
green versions.