replica0 header should not include `FromMCReplica` key

Created by: siranipour

Currently the .dat file for replica 0 has a FromMCReplica key which doesn't make sense. The reason is because

https://github.com/NNPDF/nnpdf/blob/4d3f28f08b6cdf497c733b8488bf9fde85363f8a/validphys2/src/validphys/lhio.py#L196

Uses the last loaded header from the preceding loop

https://github.com/NNPDF/nnpdf/blob/4d3f28f08b6cdf497c733b8488bf9fde85363f8a/validphys2/src/validphys/lhio.py#L185