#274 remove vestigial lyx-related material

[CNCdreamer]

now that the lyx source files have been removed from V2.5, it's time to remove the vestigial lyx-related material

Files I believe should be removed (but this should be checked) include

docs/src/l2h.xsl
docs/src/lxx2ad.py
docs/src/lyxdep.py
docs/src/lyxparser.py
docs/src/lyxtree.py

and possibly

docs/src/xcommon.py
docs/src/xref.py
docs/src/xref.xsl

Also, there are a number of instances of the string 'lyx' (and 'LyX') in docs/src/Submakefile that, while benign, can only confuse future maintainers.

Regards,
Kent

[CNCdreamer]

Oops, it looks like there may be some more candidates for removal that I overlooked:

docs/src/links.xslt
docs/src/terms.xml
docs/src/xcommon.py
docs/src/xref.py
docs/src/xref.xsl

The proof of the pudding is for me to make all the removals I've mentioned and then try to rebuild the docs. I'll try to get to it later this weekend.