Remove ci_dynamic_pipeline_inputs feature flag

What does this MR do and why?

Removes the ci_dynamic_pipeline_inputs feature flag - feature was released in 18.7

Changes:

• Deletes the feature flag definition file
• Removes feature flag checks from GraphQL types (SpecType, ComponentType, ProjectType)
• Removes feature flag validation from Gitlab::Ci::Config::Header::Input
• Cleans up related specs by removing feature flag stubs and consolidating test cases

References

• Feature issue: #563375 (closed)
• Rollout issue: #572854
• Introduced in: !205726 (merged)

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