kustomize: Fix error while generating release manifests

This fixes the below error that occurs in `make release`:

```
kustomize build config/default > release/actions-runner-controller.yaml
Error: accumulating resources: accumulation err='accumulating resources from '../webhook': '/home/mumoshu/p/actions-runner-controller/config/webhook' must resolve to a file': recursed accumulation of path '/home/mumoshu/p/actions-runner-controller/config/webhook': accumulating resources: accumulation err='accumulating resources from 'manifests.v1beta1.yaml': evalsymlink failure on '/home/mumoshu/p/actions-runner-controller/config/webhook/manifests.v1beta1.yaml' : lstat /home/mumoshu/p/actions-runner-controller/config/webhook/manifests.v1beta1.yaml: no such file or directory': evalsymlink failure on '/home/mumoshu/p/actions-runner-controller/config/webhook/manifests.v1beta1.yaml' : lstat /home/mumoshu/p/actions-runner-controller/config/webhook/manifests.v1beta1.yaml: no such file or directory
make: *** [Makefile:156: release] Error 1
```

Ref #144

config/webhook/kustomization.yaml

@@ -1,6 +1,5 @@
 resources:
 - manifests.yaml
-- manifests.v1beta1.yaml
 - service.yaml
 
 configurations: