fix: remove MetalLB setup stage — rely on pre-installed MetalLB

MetalLB is already installed and configured on the cluster. The pipeline
no longer needs to apply IPAddressPool or L2Advertisement resources.
Removed the 'Setup MetalLB' stage and deleted the metallb overlay files.
The frontend Service type: LoadBalancer is already set, so MetalLB will
automatically assign an external IP on deployment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

k8s/overlays/on-premise/metallb/ipaddresspool.yaml

@@ -1,8 +0,0 @@
-apiVersion: metallb.io/v1beta1
-kind: IPAddressPool
-metadata:
-  name: scrum-manager-pool
-  namespace: metallb-system
-spec:
-  addresses:
-    - 192.168.108.213/32

k8s/overlays/on-premise/metallb/l2advertisement.yaml

@@ -1,8 +0,0 @@
-apiVersion: metallb.io/v1beta1
-kind: L2Advertisement
-metadata:
-  name: scrum-manager-l2
-  namespace: metallb-system
-spec:
-  ipAddressPools:
-    - scrum-manager-pool