remove variable 'etcd_ionice', because ionice removed from container image etcd:v3.4.x (#6735)

roles/etcd/templates/etcd.j2

@@ -17,8 +17,5 @@
 {% endif %}
   --name={{ etcd_member_name | default("etcd") }} \
   {{ etcd_image_repo }}:{{ etcd_image_tag }} \
-{% if etcd_ionice is defined %}
-  /bin/ionice {{ etcd_ionice }} \
-{% endif %}
   /usr/local/bin/etcd \
   "$@"