DR: get rid of `geo_postgresql['enable'] = true` step

Description

Almost at the end of the DR setup, we are being asked to add geo_postgresql['enable'] = true to gitlab.rb (documentation).

Having to update gitlab.rb at this stage feels like a why should I modify this file again??.

Can we add this line at the stage where we already add geo_secondary_role['enable'] = true on the secondary (documentation).