xen: drop kexec bits from /sys/hypervisor since kexec isn't implemented yet

I needed this to compile since there is no kexec yet in pvops kernel
  CC      drivers/xen/sys-hypervisor.o
drivers/xen/sys-hypervisor.c: In function 'hyper_sysfs_init':
drivers/xen/sys-hypervisor.c:405: error: 'vmcoreinfo_size_xen' undeclared (first use in this function)
drivers/xen/sys-hypervisor.c:405: error: (Each undeclared identifier is reported only once
drivers/xen/sys-hypervisor.c:405: error: for each function it appears in.)
drivers/xen/sys-hypervisor.c:406: error: implicit declaration of function 'xen_sysfs_vmcoreinfo_init'
drivers/xen/sys-hypervisor.c: In function 'hyper_sysfs_exit':
drivers/xen/sys-hypervisor.c:433: error: 'vmcoreinfo_size_xen' undeclared (first use in this function)
drivers/xen/sys-hypervisor.c:434: error: implicit declaration of function 'xen_sysfs_vmcoreinfo_destroy'

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