[Rails5] Disable ONLY_FULL_GROUP_BY mode for mysql

What does this MR do?

Explicitly disables ONLY_FULL_GROUP_BY mysql connection option. In Rails 5 this option is enabled by default which causes that some of our SQL queries fail. This MR also removes workaround used for fixing this issue (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20256)

What are the relevant issue numbers?

gitlab-ee!7086

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides