DNS service is installed for shoots without DNS domain

[rfranzke]

What happened:
Shoots that don't specify a DNS domain (.spec.dns = nil) or that are scheduled to a seed that is tainted with "DNS disabled" still get the DNS service.

What you expected to happen:
The DNS service shoot only be installed for shoots with a DNS domain.

How to reproduce it (as minimally and precisely as possible):
Create a shoot with .spec.dns=nil and register the DNS extension (globally enabled). See it getting the DNS service pod.

• Gardener version (if relevant): v1.0.4
• Extension version: v1.3.0
• Kubernetes version (use kubectl version): doesn't matter
• Cloud provider or hardware configuration: doesn't matter