more simplification of forced-eviction from page-in function

@agorrod, this change gets rid of the WT_PAGE_EVICT_FORCE flag entirely (its only effect, AFAICT, is to wake up the eviction server once per forced page eviction).

If you don't like this change, feel free to discard it.  (I'm pushing it to you, you have a better sense of what's needed in eviction than I do.)