Neither Firefox nor Thunderbird use any of rdfISerializer,
NS_RDFNTRIPLES_SERIALIZER_CID, and
"@mozilla.org/rdf/serializer;1?format=ntriples".
https://dxr.mozilla.org/addons/source/ shows three addons that use them, but
only one does so meaningfully: ArchView. (The others mention them as part of
long lists of XPCOM interfaces.) According to AMO, ArchView hasn't been updated
since August 2008, it has only 153 users, it doesn't work in Firefox 53. Also,
judging from the code, rdfISerializer is only be used as part of debugging the
addon itself. So I think that use can be ignored.
This patch removes them. It also removes rdfTriplesSerializer.cpp.
dcc7c49325