gitlab-ci: remove the "linux-old" job

The "linux-old" job was historically testing against the oldest
supported LTS release of Ubuntu. But with c85bcb5 (gitlab-ci: switch
from Ubuntu 16.04 to 20.04, 2024-10-31) it has been converted to test
against Ubuntu 20.04, which already gets exercised in a couple of other
CI jobs. It's thus not adding any significant test coverage.

Drop the job.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

.gitlab-ci.yml

@@ -36,9 +36,6 @@ test:linux:
       fi
   parallel:
     matrix:
-      - jobname: linux-old
-        image: ubuntu:20.04
-        CC: gcc
       - jobname: linux-sha256
         image: ubuntu:latest
         CC: clang