fix: drop orphaned image-less sidekiq from compose.smtpauth.yml

The official-image migration removed the separate sidekiq service from
compose.yml (sidekiq now runs inside the official discourse app image). The
compose.smtpauth.yml overlay still declared a sidekiq: block (smtp env +
smtp_password secret, no image), leaving a dangling image-less service in the
smtpauth-merged compose -> abra recipe lint R011 fail (invalid reference
format) and a broken SMTP-auth deploy. The app: override already carries the
DISCOURSE_SMTP_PASSWORD_FILE env + smtp_password secret (and compose.yml app
has all DISCOURSE_SMTP_* vars), so the sidekiq override is redundant. Drop it.

compose.smtpauth.yml

@@ -8,12 +8,6 @@ services:
     secrets:
       - smtp_password
 
-  sidekiq:
-    environment:
-      - DISCOURSE_SMTP_PASSWORD_FILE=/var/run/secrets/smtp_password
-    secrets:
-      - smtp_password
-
 secrets:
   smtp_password:
     external: true