Remove unused ReadOnlyReport.append

[Swatinem]

This method seemed to have been intended to warn about wrong usage of ReadOnlyReport.

I checked both in production logs, as well as scanning through the code, and it looks like we are not misusing the this report class in such a way. So it is safe to remove this warning.

[codspeed-hq]

CodSpeed Performance Report

Merging #526 will create unknown performance changes

_{Comparing swatinem/readonly-append (6320b19) with main (aba4c5c)}

Summary

⚠️ No benchmarks were detected in both the base of the PR and the PR.\

[giovanni-guidini]

Would we benefit from throwing an exception here instead of hoping that we will keep not misusing the ReadOnlyReport?

EDIT ah no I see ReadOnlyReport is not a subclass of any other report, so removing it would cause an exception anyway