package/docker-engine: drop <pkg>_SRC_SUBDIR variable

With the go.mod integration, the DOCKER_ENGINE_SRC_SUBDIR has become
useless, drop it.

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

package/docker-engine/docker-engine.mk

@@ -11,7 +11,6 @@ DOCKER_ENGINE_LICENSE = Apache-2.0
 DOCKER_ENGINE_LICENSE_FILES = LICENSE
 
 DOCKER_ENGINE_DEPENDENCIES = host-pkgconf
-DOCKER_ENGINE_SRC_SUBDIR = github.com/docker/docker
 DOCKER_ENGINE_GOMOD = github.com/docker/docker
 
 DOCKER_ENGINE_LDFLAGS = \