xen/kexec: remove spinlock now that all KEXEC hypercall ops are protected at the...

xen/kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

The spinlock in kexec_swap_images() was removed as
this function is only reachable on the kexec hypercall, which is
now protected at the top-level in do_kexec_op_internal(),
thus the local spinlock is no longer necessary.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>