Fix failing when the sole dovecot user is updated.

When a single dovecot user exists and their password is updated,
the `grep -v` command intended to remove the user's old password
will not match any lines and fail, causing the entire script to fail.

docker/add-user.sh

@@ -34,11 +34,7 @@ ENCPASS=$(doveadm pw -u "${EMAIL}" -p "${PASSWORD}")
 # Edit dovecot users: remove user if it exits.
 mkdir -p /data/dovecot
 touch /data/dovecot/users
-if grep -q "^${EMAIL}:" /data/dovecot/users; then
-	cp /data/dovecot/users /data/dovecot/users.old
-	grep -v "^${EMAIL}:" /data/dovecot/users.old \
-		> /data/dovecot/users
-fi
+sed -i "/^${EMAIL}:/d" /data/dovecot/users
 
 # Edit dovecot users: add user.
 echo "${EMAIL}:${ENCPASS}::::" >> /data/dovecot/users