[DCR] Remove old feature flags
What does this MR do and why?
- This MR removes the following FFs for DCR:
duo_code_review_custom_instructions
duo_code_review_claude_4_0_rollout
- And removes the usage of
use_claude_code_completion
fromee/lib/gitlab/llm/prompt_resolvers/review_merge_request.rb
as it's no longer needed.
MR acceptance checklist
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Related to #545820 (closed), #546438 (closed)
Edited by Kinshuk Singh