Use default GIT_STRATEGY in CI

The "clone" strategy clones the entire repository, which takes
a significant amount of time. It's not needed, because the default
"fetch" strategy will reuse the executor's local clone and clean it
before the job scripts run.

.gitlab-ci.yml

@@ -3,7 +3,6 @@ include:
   - '.gitlab/ci/jobs/*.yml'
 
 variables:
-  GIT_STRATEGY: clone
   GIT_CLONE_PATH: $CI_BUILDS_DIR/$CI_CONCURRENT_ID/$CI_PROJECT_PATH
 
 stages: