Remove flannel_cni_download_url (#10188)

Since the commit 937e64d the variable
has not been used at all.
This removes it from offline.yml which was the remaining part.

inventory/sample/group_vars/all/offline.yml

@@ -40,9 +40,6 @@
 # [Optional] Cilium: If using Cilium network plugin
 # ciliumcli_download_url: "{{ files_repo }}/github.com/cilium/cilium-cli/releases/download/{{ cilium_cli_version }}/cilium-linux-{{ image_arch }}.tar.gz"
 
-# [Optional] Flannel: If using Flannel network plugin
-# flannel_cni_download_url: "{{ files_repo }}/kubernetes/flannel/{{ flannel_cni_version }}/flannel-{{ image_arch }}"
-
 # [Optional] helm: only if you set helm_enabled: true
 # helm_download_url: "{{ files_repo }}/get.helm.sh/helm-{{ helm_version }}-linux-{{ image_arch }}.tar.gz"