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

With the go.mod integration, the <pkg>_WORKSPACE variable has become
useless, drop it.

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

package/docker-proxy/docker-proxy.mk

@@ -12,8 +12,6 @@ DOCKER_PROXY_LICENSE_FILES = LICENSE
 
 DOCKER_PROXY_DEPENDENCIES = host-pkgconf
 
-DOCKER_PROXY_WORKSPACE = gopath
-
 DOCKER_PROXY_BUILD_TARGETS = cmd/proxy
 
 define DOCKER_PROXY_INSTALL_TARGET_CMDS