Keep ONLY_FULL_GROUP_BY restriction for plugin DB connection

[MAILPOET-2630]

lib/Doctrine/ConnectionFactory.php

@@ -55,7 +55,6 @@ class ConnectionFactory {
   private function getDriverOptions($timezoneOffset, $charset, $collation) {
     $driverOptions = [
       "@@session.time_zone = '$timezoneOffset'",
-      '@@session.sql_mode = REPLACE(@@sql_mode, "ONLY_FULL_GROUP_BY", "")',
       // We need to use CONVERT for MySQL 8, Maria DB bug which triggers #1232 - Incorrect argument type to variable 'wait_timeout`
       // https://stackoverflow.com/questions/35187378/mariadb-type-error-when-setting-session-variable
       "@@session.wait_timeout = GREATEST(CONVERT(COALESCE(@@wait_timeout, 0), SIGNED), $this->minWaitTimeout)",