Add a KASSERT to pmap_remove_write() to ensure that the page obtained
from the PV list is not a superpage where we don't expect one. An
identical KASSERT already exists in pmap_clear_modify().
This may help with debugging the panic reported in PR 250866, in which
l3 was found to be NULL. Adding this KASSERT will help narrow down the
cause of this panic the next time it occurs.