sheep: remove sha1_from_buffer()

This function is buggy yet tricky to debug. For now we can't pass following
script:

for i in `seq 0 2`; do
	sheep/sheep -D -n -z $i -p $((7000+$i)) -c local store/$i
done
sleep 1
dog/dog cluster format -c 3
qemu-img convert linux-0.2.img sheepdog:test
dog/dog vdi snapshot test -s snap
dog/dog cluster snapshot save snap1 snapshot
md5sum /tmp/store/0/obj/{007c2b2500000000,007c2b2500000001,007c2b2500000002,007c2b2500000003,007c2b2500000004}
dog/dog cluster snapshot load snap1 snapshot
md5sum /tmp/store/0/obj/{007c2b2500000000,007c2b2500000001,007c2b2500000002,007c2b2500000003,007c2b2500000004}
dog/dog vdi read test | md5sum
md5sum linux-0.2.img

-----------------------------------------

Instead of debugging of it, this patch tries to remove it since the speedup
from sha1_from_buffer is very limited because we don't have many zero filled
objects.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>