Remove self_heal_build_trace_chunk_flushing feature flag definition

What does this MR do and why?

This MR removes the self_heal_build_trace_chunk_flushing feature flag as part of the cleanup process outlined in #555523 (closed).

The feature flag has been successfully rolled out to 100% and is now stable, so we can permanently enable the feature and remove the flag complexity from the codebase.

Changes made

• Remove the YAML feature flag definition from config/feature_flags/gitlab_com_derisk/self_heal_build_trace_chunk_flushing.yml
• Remove the feature flag check from app/models/ci/build_trace_chunk.rb and permanently enable the self-healing behavior
• Update test files to remove feature flag stubbing and related test contexts:
  • spec/models/ci/build_trace_chunk_spec.rb
  • spec/workers/ci/build_trace_chunk_flush_worker_spec.rb

Related to #555523 (closed)