chore: Remove legacy ENV SASL_PASSWD (#2946)

As per deprecation notice from v11.3 release notes, and a related prior PR; this ENV is to be removed.

It's no longer considered useful, and none of the tests that configured it were actually using it for relaying anything.

test/tests/serial/mail_special_use_folders.bats

@@ -7,7 +7,6 @@ setup_file() {
   docker run -d --name mail_special_use_folders \
     -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
     -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-    -e SASL_PASSWD="external-domain.com username:password" \
     -e ENABLE_CLAMAV=0 \
     -e ENABLE_SPAMASSASSIN=0 \
     -e PERMIT_DOCKER=host \

test/tests/serial/mail_undef_spam_subject.bats

@@ -30,7 +30,6 @@ function setup() {
     -e SA_SPAM_SUBJECT="SPAM: " \
     -e VIRUSMAILS_DELETE_DELAY=7 \
     -e ENABLE_SRS=1 \
-    -e SASL_PASSWD="external-domain.com username:password" \
     -e ENABLE_MANAGESIEVE=1 \
     -e PERMIT_DOCKER=host \
     --name "${CONTAINER}" \

test/tests/serial/mail_with_mdbox.bats

@@ -7,7 +7,6 @@ setup_file() {
   docker run -d --name mail_with_mdbox_format \
     -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
     -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-    -e SASL_PASSWD="external-domain.com username:password" \
     -e ENABLE_CLAMAV=0 \
     -e ENABLE_SPAMASSASSIN=0 \
     -e DOVECOT_MAILBOX_FORMAT=mdbox \

test/tests/serial/mail_with_sdbox.bats

@@ -7,7 +7,6 @@ setup_file() {
   docker run -d --name mail_with_sdbox_format \
     -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
     -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-    -e SASL_PASSWD="external-domain.com username:password" \
     -e ENABLE_CLAMAV=0 \
     -e ENABLE_SPAMASSASSIN=0 \
     -e DOVECOT_MAILBOX_FORMAT=sdbox \