ci: remove unnecessary Docker variables from configuration

Removes unused Docker-related variables from the CI configuration to 
simplify the setup. This streamlines the CI process by eliminating 
settings that are no longer needed, enhancing clarity and maintenance.

.gitlab-ci.yml

@@ -13,11 +13,6 @@ stages:
 
 variables:
   UNBOUND_RELEASE_TAG_ONLY: true
-  DOCKER_HOST: tcp://docker:2376
-  DOCKER_TLS_CERTDIR: "/certs"
-  DOCKER_TLS_VERIFY: 1
-  DOCKER_CERT_PATH: "$DOCKER_TLS_CERTDIR/client"
-  DOCKER_DRIVER: overlay2
 
 .buildtools:
   image: buildtool/build-tools:${BUILDTOOLS_VERSION}