fixed issue with kubermatic constraints delete failing when user cluster constraint is missing

[lsviben]

What this PR does / why we need it:
Fixes an issue with kubermatic constraints delete failing when user cluster constraint is missing. It was causing such clusters to hang when being deleted because of the finalizers on the constraints.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #6597

Does this PR introduce a user-facing change?:

Fixed a bug with kubermatic constraints delete getting stuck when corresponding user cluster constraint is missing

[lsviben]

/retest

[xrstf]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 4b27cd238535ac709a5dba65b8275f965d44470c

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lsviben, xrstf

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• pkg/controller/OWNERS [xrstf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[lsviben]

/cherry-pick release/v2.16

[kubermatic-bot]

@lsviben: #6598 failed to apply on top of branch "release/v2.16":

Applying: fixed issue with kubermatic constraints delete failing because user cluster constraint is missing
Using index info to reconstruct a base tree...
M	pkg/controller/user-cluster-controller-manager/constraint-syncer/controller.go
M	pkg/controller/user-cluster-controller-manager/constraint-syncer/controller_test.go
Falling back to patching base and 3-way merge...
Auto-merging pkg/controller/user-cluster-controller-manager/constraint-syncer/controller_test.go
Auto-merging pkg/controller/user-cluster-controller-manager/constraint-syncer/controller.go
CONFLICT (content): Merge conflict in pkg/controller/user-cluster-controller-manager/constraint-syncer/controller.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 fixed issue with kubermatic constraints delete failing because user cluster constraint is missing
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release/v2.16

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.