xen/setup: Remove redundant filtering of PTE masks.

commit 7347b408 "xen: Allow
unprivileged Xen domains to create iomap pages" added a redundant
line in the early bootup code to filter out the PTE. That
filtering is already done a bit earlier so this extra processing
is not required.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>