ci: testbed: stop rewriting Debian package sources

The testbed setup rewrites the Debian mirror to archive.debian.org
before installing runtime cram dependencies. This was needed for the
old Buster container, but Debian 13 uses current mirrors and stores its
configuration in deb822 files instead of /etc/apt/sources.list.

Both focused cram jobs stop at the first rewrite before DUT bring-up:

```text
$ sudo sed -i 's|http://deb.debian.org/debian|http://archive.debian.org/debian|g' /etc/apt/sources.list 2>/dev/null
Running after_script
```

Remove both obsolete rewrites and let apt use the sources supplied by
the versioned Debian 13 testbed image.

Assisted-by: codex:gpt-5
Signed-off-by: Petr Štetiar <petr.stetiar@prplfoundation.org>
Fixes: 6792ba6b95 ("PROVISORY: ci: testbed: install minidlna and avahi packages at runtime")
References: PPW-1258

.gitlab/testbed.yml

@@ -59,8 +59,6 @@
     - sudo ip link set $TESTBED_WAN_INTERFACE up 2> /dev/null
     - sleep 10
 
-    - sudo sed -i 's|http://deb.debian.org/debian|http://archive.debian.org/debian|g' /etc/apt/sources.list 2>/dev/null
-    - sudo sed -i 's|http://deb.debian.org/debian-security|http://archive.debian.org/debian-security|g' /etc/apt/sources.list 2>/dev/null
     - sudo apt-get update && sudo apt-get install --yes --no-install-recommends minidlna avahi-daemon avahi-utils dbus
 
     - mkdir -p ~/.ssh