The changes to pmap_demote_pde_locked()'s control flow in r348476 resulted
in the loss of a KASSERT that guarded against the invalidation a wired
mapping. Restore this KASSERT.

Remove an unnecessary KASSERT from pmap_demote_pde_locked(). It guards
against a state that was already handled at the start of the function.

Reviewed by: kib
X-MFC with: r348476