x86/Xen: drop leftover VM-assist uses

Both the 4Gb-segments and the PAE-extended-CR3 one are applicable to
32-bit guests only. The PAE-extended-CR3 use, furthermore, was redundant
with the PAE_MODE ELF note anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

Link: https://lore.kernel.org/r/215515af-cfb9-3237-03ba-3312e3fa0d34@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>