docker-containerd: remove symlink to $(RUNC_SRCDIR)

docker-containerd: remove symlink to $(RUNC_SRCDIR)

The DOCKER_CONTAINERD_CONFIGURE_CMDS creates a symlink to runc's
source directory, but it does not build depend on runc, which means
that the runc package may not have been extracted/built before
docker-containerd.

Therefore, when doing a build with "make docker-containerd", this
symbolic link is broken, but it doesn't prevent docker-containerd from
building.

Therefore, this symlink is not necessary and can be dropped.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>