drivers/xen: make sys-hypervisor.c explicitly non-modular

The Kconfig currently controlling compilation of this code is:

config XEN_SYS_HYPERVISOR
       bool "Create xen entries under /sys/hypervisor"

...meaning that it currently is not being built as a module by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

Since module_init translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.  However
one could argue that fs_initcall() might make more sense here.

This change means that the one line function xen_properties_destroy()
has only one user left, and since that is inside an #ifdef, we just
manually inline it there vs. adding more ifdeffery around the function
to avoid compile warnings about "defined but not used".

In order to be consistent we also manually inline the other _destroy
functions that are also just one line sysfs functions calls with only
one call site remaing, even though they wouldn't need #ifdeffery.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>