[xml] Stepping down

[Nick Wellnhofer <wellnhofer aevum de>]

I never really asked for it but in the last years I became de-facto maintainer 
of both libxml2 and libxslt. Luckily, I was able to fund my involvement 
through Chrome VRP bug bounties and OSS-Fuzz integration rewards. Big thanks 
to Google for these outstanding programs.

Unfortunately, returns from security research are diminishing quickly and I 
see no way to obtain a minimal level of funding anymore. So I'm stepping down 
as contributor and maintainer.

Thanks to everyone who reported bugs and contributed patches!

Nick