MEDIUM: pools: use a single pool_gc() function for locked and lockless

MEDIUM: pools: use a single pool_gc() function for locked and lockless

Locked and lockless shared pools don't need to use a different pool_gc()
function because this function isolates itself during the operation, so
we do not need to rely on DWCAS nor any atomic operation in fact. Let's
just get rid of the lockless one in favor of the simple one. This should
even result in a faster execution.

The ifdefs were slightly moved so that we can have pool_gc() defined
as soon as there are global pools, this avoids duplicating the function.