Remove ci_skip_pipelines_with_fully_filtered_includes flag

What does this MR do and why?

This MR removes the ci_skip_pipelines_with_fully_filtered_includes feature flag.

The flag changed what happens when include:rules: filters out every include: in a CI config. The config then has zero visible jobs. The old behaviour saved a failed config_error pipeline. The new behaviour treats it as filtered_by_rules and saves no pipeline.

The flag was fully enabled on production before this removal, so the new behaviour is already live. This MR just makes it permanent.

Changes:

• Drop the Feature.enabled? check in empty_because_includes_filtered? in lib/gitlab/ci/pipeline/chain/config/process.rb.
• Delete config/feature_flags/gitlab_com_derisk/ci_skip_pipelines_with_fully_filtered_includes.yml.
• Remove the disabled-flag spec context from spec/lib/gitlab/ci/pipeline/chain/config/process_spec.rb.

Related links

• Cleanup issue: #619278 (closed)
• Rollout issue: #609470 (closed)
• Feature issue: #591780 (closed)
• Introduced by: !246515 (merged)

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Related to #619278 (closed)