x86/entry/32: Simplify CONFIG_XEN_PV build dependency

With 32-bit Xen PV support gone, the following commit is not needed
anymore:

  a4c0e91d ("x86/entry/32: Fix XEN_PV build dependency")

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200815100641.26362-5-jgross@suse.com