Remove cache_ci_build_trace_chunk_fog_connection feature flag

Removes the cache_ci_build_trace_chunk_fog_connection feature flag and makes the cached-connection behavior the permanent default for Ci::BuildTraceChunks::Fog.

The flag guarded reusing one Fog::Storage connection per process (keyed by credentials) instead of rebuilding the connection and re-fetching googleauth credentials on nearly every job-status update. It rolled out in 19.4 as a gitlab_com_derisk flag (default disabled) and is now fully enabled on production after proving stable.

This also drops the IAM instance-profile guard. That guard skipped the process-wide cache because fog-aws refreshed IAM credentials in place without synchronization, which raced under concurrent requests. The fix shipped in fog-aws 3.33.3 (https://github.com/fog/fog-aws/pull/759), the version now pinned in Gemfile.lock, so the shared connection is safe for all credential types.

Changes: removed the Feature.enabled? check and the IAM guard from connection, deleted the flag YAML file, and removed the obsolete spec contexts.

Related issue: #623936 (closed)