chore: Merge helpers/sasl.sh into helpers/relay.sh (#2605)

This helper was to support an earlier ENV for SASL auth support. When extracting logic into individual helpers, it was assumed this was separate from relay support, which it appears was not the case.

---

The `SASL_PASSWD` ENV is specified in tests but no longer used. There is no `external-domain.com` relay configured or tested against anywhere in the project.

The ENV was likely used in tests prior to improved relay support that allowed for adding more than a single set of relay credentials.

---

It likewise has no real relevance anywhere else outside of `relay.sh` as it's the only portion of code to operate with it.

It's only relevant for SASL auth as an SMTP client, not the SMTP server (`smtpd`) SASL support that is delegated to Dovecot. Functionality has been completely migrated into `relay.sh` as a result.

Documentation is poor for this ENV, it is unlikely in wide use? Should consider for removal.

---

The ENV has been dependent upon `RELAY_HOST` to actually enable postfix to use `/etc/postfix/sasl_passwd`, thus not likely relevant in existing setups?

---

Migrate `/etc/postfix/sasl_passwd` check from `tests.bats` as it belongs to relay tests.

target/scripts/startup/setup-stack.sh

@@ -764,21 +764,6 @@ function _setup_postfix_override_configuration
   fi
 }
 
-function _setup_postfix_sasl_password
-{
-  _log 'debug' 'Setting up Postfix SASL Password'
-
-  # support general SASL password
-  _sasl_passwd_create
-
-  if [[ -f /etc/postfix/sasl_passwd ]]
-  then
-    _log 'trace' 'Loaded SASL_PASSWD'
-  else
-    _log 'debug' "SASL_PASSWD was not provided - '/etc/postfix/sasl_passwd' not created"
-  fi
-}
-
 function _setup_postfix_relay_hosts
 {
   _setup_relayhost