Drop stalwart mail proxying from the haproxy edge (#491)

Stalwart was only ever a test deployment. The daemon is dead on all three
backend VMs and nothing public depends on it — `unkin.net` MX points at Google —
so the edge is proxying mail to nowhere and the tcp frontends make `defaults`
emit 20 spurious HTTP-mode warnings.

- Drop the `fe_smtp`, `fe_submission`, `fe_imap` and `fe_imaps` frontends.
- Drop the five `be_stalwart_*` backends and their map entries in `fe_http.map`/`fe_https.map`.
- Drop the now-unused 25/143/587/993 Service and container ports.

`haproxy -c` on the rendered config: exit 0, 0 warnings (was 20), 0 alerts.

Reviewed-on: #491
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/haproxy/service.yaml

@@ -10,7 +10,7 @@ metadata:
 spec:
   type: LoadBalancer
   loadBalancerIP: "198.18.199.1"
-  # Source IP must survive for acl_internalsubnets, X-Real-IP and SMTP.
+  # Source IP must survive for X-Real-IP.
   externalTrafficPolicy: Local
   # Pins a client to one replica, standing in for the dropped stick-table peers.
   sessionAffinity: ClientIP
@@ -25,19 +25,3 @@ spec:
       port: 443
       protocol: TCP
       targetPort: https
-    - name: smtp
-      port: 25
-      protocol: TCP
-      targetPort: smtp
-    - name: imap
-      port: 143
-      protocol: TCP
-      targetPort: imap
-    - name: submission
-      port: 587
-      protocol: TCP
-      targetPort: submission
-    - name: imaps
-      port: 993
-      protocol: TCP
-      targetPort: imaps