CLEANUP: pools: remove the now unused pool_is_crowded()

CLEANUP: pools: remove the now unused pool_is_crowded()

This function was renderred obsolete by commit a0b5831ee ("MEDIUM: pools:
centralize cache eviction in a common function") which replaced its last
call inside the loop with a single call out of the loop to pool_releasable()
as introduced by commit 91a8e28f9 ("MINOR: pool: add a function to estimate
how many may be released at once"). Let's remove it before it becomes wrong
and used again.