remove windows registration command from k3s custom clusters

[slickwarren]

Setup

• Rancher version: v2.6.9-rc3
• Browser type & version: chrome (any)

Describe the bug

k3s does not support windows clusters. However when provisioning a custom k3s cluster through rancher, after registering the first linux node a windows registration command pops up for the k3s cluster

To Reproduce

• provision a k3s custom cluster through rancher
• register 1 linux node to check if the windows registration command appears

Result the windows registration command appears

Expected Result

windows is not supported on k3s so it should not appear for custom k3s clusters