Really stop supplying the zuul url to ggp

The ggp change propagated to bare slave images long ago from
I8c88d09c7a311edf526cffc730afd4471895eeae but this got missed in
cleanup or re-introduced for some reason. After this, it should be
safe to remove the corresponding backward-compatibility conditional
in the slave script.

Change-Id: I937945965cf101b16da53fc399cea0fcfc5ff233
Merge "Really stop supplying the zuul url to ggp"