Strip throughput rates if all are `?`

Thanks for critcmp. My benches don't tend to have throughput figures, I get two columns of `? B/sec`.

It's not the worst thing having them, but maybe these could be auto-stripped away when they're all `?`?

Example:
```
group master new
----- ------ ---
no_cache_render_3_medium_sections_fully 1.29 1647.1±18.17µs ? B/sec 1.00 1277.4±17.63µs ? B/sec
no_cache_render_v_bottom_1_large_section_partially 1.62 7.6±0.02ms ? B/sec 1.00 4.7±0.02ms ? B/sec
no_cache_render_v_center_1_large_section_partially 1.66 7.7±0.02ms ? B/sec 1.00 4.7±0.04ms ? B/sec
```