Remove ci_lazy_predefined_variables feature flag

What does this MR do and why?

Removes the ci_lazy_predefined_variables feature flag, making lazy evaluation of expensive predefined CI variables (Gitaly and DB queries) the default behavior for improved pipeline creation performance.

Changes:

• Deletes the feature flag YAML definition
• Removes feature flag checks and eager evaluation fallbacks from MergeRequest#merge_request_approval_variables and Variables::Builder::Pipeline
• Simplifies append_variable to always use lazy evaluation
• Updates specs to remove feature flag toggle contexts

Related issues

• Feature issue: #410143 (closed)
• Rollout issue: #594448 (closed)

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