Remove unused internal utility: `calls_sys()`

[github-actions[bot]]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if d764e7832ea650674a0044c460612cf18cbade65 is merged into main:

• :heavy_check_mark:cache_applying: 157ms -> 159ms [-0.97%, +3.31%]
• :heavy_check_mark:cache_recording: 519ms -> 522ms [-0.27%, +1.26%]
• :heavy_check_mark:without_cache: 991ms -> 992ms [-0.69%, +0.82%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[lorenzwalthert]

Code with least bugs is deleted code 😀