[DCR] Remove old feature flags
What does this MR do and why?
- This MR removes the following FFs for DCR:
duo_code_review_custom_instructionsduo_code_review_claude_4_0_rollout
- And removes the usage of
use_claude_code_completionfromee/lib/gitlab/llm/prompt_resolvers/review_merge_request.rbas it's no longer needed.
MR acceptance checklist
Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to #545820 (closed), #546438 (closed)
Edited by Kinshuk Singh