package/docker-containerd: drop <pkg>_WORKSPACE variable

With the go.mod integration, the <pkg>_WORKSPACE variable is no longer
needed, drop it.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/docker-containerd/docker-containerd.mk

@@ -9,7 +9,6 @@ DOCKER_CONTAINERD_SITE = $(call github,containerd,containerd,v$(DOCKER_CONTAINER
 DOCKER_CONTAINERD_LICENSE = Apache-2.0
 DOCKER_CONTAINERD_LICENSE_FILES = LICENSE
 
-DOCKER_CONTAINERD_WORKSPACE = vendor
 DOCKER_CONTAINERD_GOMOD = github.com/containerd/containerd
 
 DOCKER_CONTAINERD_LDFLAGS = \