Build a single DEBIAN_VER variant

Since the introduction of Debian Buster with support for
both Java8 and Java11, the generation of multiple base
images isn't needed anymore.

This is a follow-up of Change-Id: Ifda36480.

Change-Id: I85bcc6275d67978e9609c8c535bc1649eac64766
diff --git a/jenkins-docker/slave-debian/Makefile b/jenkins-docker/slave-debian/Makefile
index e4ef2fb..d89181d 100644
--- a/jenkins-docker/slave-debian/Makefile
+++ b/jenkins-docker/slave-debian/Makefile
@@ -2,30 +2,21 @@
 IMAGE=gerritforge/gerrit-ci-slave-debian
 MAVEN_VER=3.6.3
 GIT_VER=2.26.2
+DEBIAN_VER=buster
 
-build: build_buster
-
-publish: publish_buster
-
-clean: clean_buster
-
-build_buster publish_buster clean_buster: DEBIAN_VER=buster
-
-publish_buster: build_buster
-
-build_buster:  id_rsa.pub
+build:  id_rsa.pub
 	@echo "Building Docker for Debian:$(DEBIAN_VER)"
 	@echo "========================================"
 	cat Dockerfile | DEBIAN_VER=$(DEBIAN_VER) MAVEN_VER=$(MAVEN_VER) GIT_VER=$(GIT_VER) envsubst > Dockerfile-$(DEBIAN_VER)
 	docker build --no-cache=$(NO_CACHE) -f Dockerfile-$(DEBIAN_VER) -t ${IMAGE}:$(DEBIAN_VER) .
 
-publish_buster:
+publish:
 	docker push ${IMAGE}:$(DEBIAN_VER)
 
 id_rsa.pub: ../master/id_rsa.pub
 	cp $? $@
 
-clean_buster:
+clean:
 	docker rmi -f ${IMAGE}
 	-rm -f id_rsa*