Removed SWAPOUT_WRITING assertion from storeSwapMetaBuild().

Removed SWAPOUT_WRITING assertion from storeSwapMetaBuild().

I do not see any strong dependency of that code on that state and we
need to be able to build swap metadata when updating a stale entry
(which would not normally be in the SWAPOUT_WRITING state).

The biggest danger is that somebody calls storeSwapMetaBuild() when the
entry metadata is not yet stable. I am not sure we have a way of
detecting that without using something as overly strong as
SWAPOUT_WRITING.