Fix finalizer removal on interrupt

[timebertt]

How to categorize this PR?

/area robustness ops-productivity
/kind bug
/priority normal

What this PR does / why we need it:

When terraformer is interrupted during terraform destroy execution (i.e. because of a timeout in the infrastructure actuator), it won't remove its finalizer from terraform resources (configmaps and secret), even if the terraform execution was successful and no infrastructure resources are left.
Leaking the finalizers will lead to manual ops effort as it will block the shoot's namespace in the seed to be stuck in termination.

Special notes for your reviewer:

Should be cherry-picked to release-v2.1.

Release note:

A bug was fixed that caused terraform to leak its finalizer on ConfigMaps and Secrets in case of an interrupt during `terraform destroy`.

[timebertt]

/test

[testmachinery]

Testrun: e2e-znr8q
Workflow: e2e-znr8q-wf
Phase: Succeeded

+--------------+--------------+-----------+----------+
|     NAME     |     STEP     |   PHASE   | DURATION |
+--------------+--------------+-----------+----------+
| pod-e2e-test | pod-e2e-test | Succeeded | 3m24s    |
+--------------+--------------+-----------+----------+

[vpnachev]

/lgtm