This filter will remove all peaks from -mz_tolerance to +mz_tolerance relative to the precursor_mz, but is supposed to keep the actual precursor-m/z peak.

I cannot really remember what use case this was for. (Please mention if you do)
And, more importantly, it is not very robust in its current form. It would indeed keep the precursor in, but only if mzs != precursor_mz, so any tiniest difference, including float rounding errors leads to the removal of that peak.