xen/arm: p2m: Re-implement p2m_remove_using using p2m_set_entry

xen/arm: p2m: Re-implement p2m_remove_using using p2m_set_entry

The function p2m_insert_mapping can be re-implemented using the generic
function p2m_set_entry.

Also drop the operation REMOVE in apply_* as nobody is using it anymore.
Note that the functions could have been dropped in one go at the end,
however I find easier to drop the operations one by one avoiding a big
deletion in the patch that converts the last operation.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabelini <sstabellini@kernel.org>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>