lint: use shell only when shell functionality is required

Fix ansible-lint 305 error:

[305] Use shell only when shell functionality is required
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

infrastructure-playbooks/purge-cluster.yml

@@ -732,7 +732,9 @@
     listen: "remove data"
 
   - name: umount osd data partition
-    shell: umount {{ item }}
+    mount:
+      path: "{{ item }}"
+      state: unmounted
     with_items: "{{ mounted_osd.stdout_lines }}"
     listen: "remove data"