arm/xen: Drop pte_mfn and mfn_pte

arm/xen: Drop pte_mfn and mfn_pte

They are not used in common code expect in one place in balloon.c which is
only compiled when Linux is using PV MMU. It's not the case on ARM.

Rather than worrying how to handle the 64KB case, drop them.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 5031612b5eaf15cb6471c6e936a515090810c8f1)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>