sks acc tests: remove an unnecessary check

[sauterp]

Description

We remove a check that waits for an update to an SKS cluster to happen. It's not necessary (anymore) as the provider does a similar check before.

Checklist

• Changelog updated (under Unreleased block)
• Acceptance tests OK
• For a new resource, datasource or new attributes: acceptance test added/updated

Testing

Ran the test locally and it failed due to a known issue not related to this change:

=== RUN   TestAccResourceSKSCluster
    resource_exoscale_sks_cluster_test.go:181: Step 2/3 error: Error running apply: exit status 1

        Error: context deadline exceeded

          with exoscale_sks_cluster.test,
          on terraform_plugin_test.tf line 6, in resource "exoscale_sks_cluster" "test":
           6: resource "exoscale_sks_cluster" "test" {

--- FAIL: TestAccResourceSKSCluster (401.42s)
FAIL
exit status 1
FAIL	github.com/exoscale/terraform-provider-exoscale/exoscale	401.427s