libxc: remove xc_ptrace

libxc: remove xc_ptrace

It has been unused since 21732:eb34666befcc. There was no response to
my RFC regarding its removal
http://marc.info/?l=xen-devel&m=128170404422822 and unfortunately we
have not been able to trace down all copyright holders of this code
for the purposes of relicensing libxc.

The code will be available in mercurial if we need to retrieve it in
the future.

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