Additional fix to installer CSR signername and webhook

It seems that including the signerName in RBAC is optional in 1.17
(and before?), but required as of 1.18.

Including signerName in the CSR itself, is optional in 1.18 but
NOT supported in 1.17 or earlier.

Hence, removing signerName from the CSR creation again. This may need
to be revisited later.

integration/manifests/webhook/webhook-create-signed-cert.sh

@@ -86,7 +86,6 @@ spec:
   groups:
   - system:authenticated
   request: $(cat ${tmpdir}/server.csr | base64 | tr -d '\n')
-  signerName: kubernetes.io/legacy-unknown
   usages:
   - digital signature
   - key encipherment