fix(t8s-cluster): remove unnecessary require for openstackImageNamePrefix (teutonet#1229)

this is not needed, as the type is `string` anyways and set to an empty
string by default

Author: cwrau

charts/t8s-cluster/values.schema.json

@@ -245,7 +245,6 @@
     "metadata",
     "version",
     "controlPlane",
-    "openstackImageNamePrefix",
     "nodePools"
   ],
   "additionalProperties": false,