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

Since the go.mod integration, the <pkg>_WORKSPACE variable is useless,
so drop it.

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/docker-cli/docker-cli.mk

@@ -6,7 +6,6 @@
 
 DOCKER_CLI_VERSION = 19.03.11
 DOCKER_CLI_SITE = $(call github,docker,cli,v$(DOCKER_CLI_VERSION))
-DOCKER_CLI_WORKSPACE = gopath
 
 DOCKER_CLI_LICENSE = Apache-2.0
 DOCKER_CLI_LICENSE_FILES = LICENSE