BK: remove command because it's now unnecessary (#2811)
BK: remove command because it's now unnecessary (#2811)

build_scion_img actually calls docker.sh which creates docker/_build/scion.stamp
Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion