tests/docker: remove the Debian base images

We no longer use these in any of our images. Clean-up the remaining
comments and documentation that reference them and remove from the
build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-31-alex.bennee@linaro.org>

.gitlab-ci.d/container-core.yml

@@ -10,8 +10,3 @@ amd64-fedora-container:
   extends: .container_job_template
   variables:
     NAME: fedora
-
-amd64-debian10-container:
-  extends: .container_job_template
-  variables:
-    NAME: debian10