Remove ReadOnly from disk structs

[zliang-akamai]

📝 Description

This is no longer a valid attribute of the structs.

✔️ How to Test

Automated Testing

make ARGS="-run TestInstance_Disk" fixtures

Manual Testing

package main

import (
	"context"
	"os"

	"github.com/linode/linodego"
)

func main() {
	ctx := context.Background()

	client := linodego.NewClient(nil)
	client.SetToken(os.Getenv("LINODE_TOKEN"))

	linodeClient.CreateInstanceDisk(
		context.Background(),
		123456, // your Linode ID
		linodego.InstanceDiskCreateOptions{
			Label: "somedisk",
			Size: 500,
		},
	)
}

[lgarber-akamai]

LGTM, thanks for fixing this!