exp/draw: remove Border function.
exp/draw: remove Border function.

It was only used by exp/4s, and even if it is general purpose, I think
it belongs in a graphics library atop exp/draw, not in exp/draw itself.

R=rsc
CC=golang-dev
http://codereview.appspot.com/3705041

ssrl/go,gnanderson/go,apparentlymart/golang,tv42/old-go,h8liu/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,dsriram/golang,ssrl/go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,h8liu/golang,apparentlymart/golang,h8liu/golang,apparentlymart/golang,h8liu/golang,dsriram/golang,gnanderson/go,gnanderson/go,h8liu/golang,h8liu/golang,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,dsriram/golang,pombredanne/go-deleteme,ssrl/go,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,ssrl/go,gnanderson/go,h8liu/golang,tv42/old-go,tv42/old-go,dsriram/golang,ssrl/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go