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/deployment.yaml

@@ -50,7 +50,7 @@ spec:
             readOnlyRootFilesystem: true
             capabilities:
               drop: [ALL]
-              # Frontends bind 25/80/143/443/587; the dst_port ACLs need the real ports.
+              # Frontends bind 80 and 443; the dst_port ACLs need the real ports.
               add: [NET_BIND_SERVICE]
           ports:
             - name: http
@@ -59,18 +59,6 @@ spec:
             - name: https
               containerPort: 443
               protocol: TCP
-            - name: smtp
-              containerPort: 25
-              protocol: TCP
-            - name: imap
-              containerPort: 143
-              protocol: TCP
-            - name: submission
-              containerPort: 587
-              protocol: TCP
-            - name: imaps
-              containerPort: 993
-              protocol: TCP
             - name: health
               containerPort: 8404
               protocol: TCP