Change defining mutation for XBC.1.6.1 from ORF1b:N498I to ORF8:P36L

In #1948, @oobb45729 raised that ORF1b:N498I seems to
be an artefact in sequences from Australia-SA

This does look to be the case, it is very homoplasic in XBC.1.6

Redefining XBC.1.6.1 to be defined by ORF8:P36L fixes this issue

@AngieHinrichs may want to apply branch specific masking on 14960
for XBC.1.6, or maybe even mask 14960 altogether given it's an
artefact in a lot of Australia-SA sequences

Author: corneliusroemer