amd64: Eliminate a redundant test from pmap_enter_object()

The call to pmap_allow_2m_x_page() in pmap_enter_object() is redundant.
Specifically, even without the call to pmap_allow_2m_x_page() in
pmap_enter_object(), pmap_allow_2m_x_page() is eventually called by
pmap_enter_pde(), so the outcome will be the same. Essentially,
calling pmap_allow_2m_x_page() in pmap_enter_object() amounts to
"optimizing" for the unexpected case.

Reviewed by: kib
MFC after: 1 week