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

sys/amd64/include/md_var.h

@@ -34,7 +34,6 @@
 
 #include <x86/x86_var.h>
 
-extern	char	ctx_switch_xsave[];
 extern  uint64_t *vm_page_dump;
 
 /* XXX */