mailserver: decommission SendGrid

Remove leftover SendGrid references after the Brevo migration was completed:

- Delete TF `cloudflare_record.mail_domainkey` (TXT at `s1._domainkey`,
  SendGrid-era DKIM, hidden behind the SendGrid CNAME but would re-emerge
  once the CNAME is removed).
- Clean up commented-out `smtp.sendgrid.net` relayhost references and the
  `# For sendgrid` comment on `sasl_passwd` in the mailserver module.

DNS records deleted out-of-band (not TF-managed):
- CF: `s1._domainkey CNAME` + `s2._domainkey CNAME` → sendgrid.net (manual entries)
- Technitium internal `viktorbarzin.me`: `em7107`, `s1._domainkey`,
  `s2._domainkey` CNAMEs → sendgrid.net

Verified end-to-end mail flow unaffected (Brevo outbound + IMAP receive,
roundtrip 20.4s — identical to baseline). Active DKIM (`mail._domainkey`
local + `brevo1/brevo2._domainkey` Brevo) untouched.

stacks/mailserver/modules/mailserver/variables.tf

@@ -2,7 +2,6 @@
 # see defaults - https://github.com/docker-mailserver/docker-mailserver/blob/master/target/postfix/main.cf
 variable "postfix_cf" {
   default = <<EOT
-#relayhost = [smtp.sendgrid.net]:587
 relayhost = [smtp-relay.brevo.com]:587
 smtp_sasl_auth_enable = yes
 smtp_sasl_password_maps = hash:/etc/postfix/sasl/passwd