docs: remove some fatally out of date documentation

docs: remove some fatally out of date documentation

I think these are better off deleted than remaining to confuse people.

docs/misc/blkif-drivers-explained.txt:

 - Talks about Xen 2.0 beta, talks about the old pre-xenstored IPC mechanism.

docs/misc/cpuid-config-for-guest.txt:

 - Doesn't really say anything, in particular doesn't actually describe how to
   configure CPUID.

docs/misc/hg-cheatsheet.txt:

 - Not out of date per-se wrt mercural but talk about Xen 2.0 and gives URLs
   under www.cl.cam.ac.uk. Talks a lot about bitkeeper. Given that mercurial is
   hardly unusual anymore I think there must be better guides out there so this
   one is not worth resurecting.

docs/misc/network_setup.txt:

 - This is more comprehensively documented on the wiki these days.

docs/misc/VMX_changes.txt:

 - Is basically a changelog from the initial implementation of VMX in 2004.

I'm not sure about some of the other docs, but these ones seemed fairly
obvious.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>