Remove calls to pmap_clear_modify() in the swap_pager, fixing a kernel panic.

Remove calls to pmap_clear_modify() in the swap_pager, fixing a kernel panic.
The modify bit is cleared by the pageout code's call to vm_page_protect()
and does not need to be cleared again.

The pmap code cannot safely access non-kernel pmaps from the interrupt
service routine it uses to process the asynchronous I/O completion.  These
non-kernel pmaps are present because the pageout code protects the page
with VM_PROT_READ (which is desireable) rather then VM_PROT_NONE.