ci: drop redundant MESA_IMAGE

It's already set by the extended `.set-image` 8 lines further up.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39923>

.gitlab-ci/container/gitlab-ci-inc.yml

@@ -20,8 +20,6 @@
   extends:
     - .set-image
     - .incorporate-base-tag+templates-commit
-  variables:
-    MESA_IMAGE: "$CI_REGISTRY_IMAGE/${MESA_IMAGE_PATH}:${FDO_DISTRIBUTION_TAG}"
 
 .container:
   stage: container