amd64 pmap: simplify recent changes to pmap_enter_pde()

When pmap_enter_pde() removes an existing 2MB page mapping within the
kernel address space, do not remap the kernel page table page since it
will shortly be unmapped. Simply zero it.

Eliminate an unnecessary conditional refcount decrement on what must be
a kernel page table page. (We only utilize this refcount on user-space
page table pages.)

Convert an assertion from a panic to a KASSERT.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51235