bench(rustfs): remove the fake s3_operations benchmark
rustfs/benches/s3_operations.rs never measured S3: all three groups
(put_object/get_object/list_objects) only black_box(data.len())/black_box(count)
with a 'In a real benchmark, this would call the actual S3 client' comment. It
gave a false 'S3 operations have benchmark coverage' signal and was dead weight
on rustfs/Cargo.toml.
Delete the file and its [[bench]] entry. S3-face performance is covered solely
by the warp A/B gate (performance-ab.yml); the runbook scope note now says so
explicitly. Micro-benchmarks stay at the function level (perf-8).
Refs rustfs/backlog#1152 (perf-9).
