erp5: stop disabling derived_merge=off optimiser.

it is no longer required with the latest ERP5 code where we have explicit ORDER BY in the outer query.
also drop it in generic_mysql too.

slapos/recipe/generic_mysql/template/my.cnf.in

@@ -8,10 +8,6 @@
 # Loud fail is really required in such case.
 sql-mode="NO_ENGINE_SUBSTITUTION"
 
-# Workaround for https://bugs.launchpad.net/maria/+bug/985828
-# that causes wrong result in Resource_zGetInventoryList etc.
-optimizer_switch = derived_merge=off
-
 skip-show-database
 %(networking)s
 socket = %(socket)s

stack/erp5/buildout.cfg

@@ -180,7 +180,7 @@ md5sum = bc6048b85b410693e60e5a77399dd1b7
 [template-my-cnf]
 <= download-base
 filename = my.cnf.in
-md5sum = dd779e54d22105702aa72cadc994d957
+md5sum = 9ae79a450f9760072ccbc8c9b2d0c377
 
 [template-mariadb-initial-setup]
 <= download-base

stack/erp5/my.cnf.in

@@ -9,10 +9,6 @@
 # Loud fail is really required in such case.
 sql_mode="NO_ENGINE_SUBSTITUTION"
 
-# Workaround for https://bugs.launchpad.net/maria/+bug/985828
-# that causes wrong result in Resource_zGetInventoryList etc.
-optimizer_switch = derived_merge=off
-
 skip_show_database
 {% set ip = parameter_dict.get('ip') -%}
 {% if ip -%}