Delete DefaultRuntimeName variable
Delete DefaultRuntimeName variable

It is no longer used or needed (see docker/docker#23775)

Signed-off-by: Kenfe-Mickael Laventure <10e26728841e39109c2809b6029d4b0763bd81d3@gmail.com>

thaJeztah/engine-api,jwhonce/engine-api,vdemeester/engine-api,docker/engine-api