Remove redundant ctx_switch_xsave declaration in sys/amd64/include/md_var.h

This variable was added to sys/x86/include/x86_var.h recently.

This unbreaks building kernel source that #includes both md_var.h and x86_var.h
with gcc 4.2.1 on amd64

Differential Revision: https://reviews.freebsd.org/D4686
Reviewed by: kib
X-MFC with: r291949
Sponsored by: EMC / Isilon Storage Division