Describe the bug
Remove the ability for cluster to drain and cordon nodes in readonly mode

To Reproduce
run:
k9s --readonly

list nodes

Use
Drain, Conrdon, Uncordon

Expected behavior

The tasks should be hidden.

Versions (please complete the following information):

• OS: Windows 7 Pro
• K9s: 0.21.7 [2856]
• K8s: v1.17.5