Removing ci_variables_complex_expressions feature flag and deprecated code branches

What does this MR do?

This MR removes the feature flag for conjunction and disjunction operators in CI config variables entries.

The feature flag was also around new regex tokenzing behavior and parsing algorithm for variables: statements, both of which have the old code completely removed here.

The feature flag has been set to on by default and running for several weeks, one issue popped up right away when we deployed it and has been fixed. No new issues have been reported since.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers