Remove restrict user defined variables from CI/CD settings

What does this MR do and why?

This commit removes the restrict_user_defined_variables field from the project_ci_cd_settings table. The field was ignored in 18.1. It is no longer used because it has been replaced the functionality offered by ci_pipeline_variables_minimum_override_role.

Changelog: removed

Issue: Drop restrict_user_defined_variables (#549586 - closed)

MR that ignored the field: Deprecate restrict user defined variables inter... (!189388 - merged)