CLEANUP: pools: move the lock to the only __pool_get_first() that needs it

CLEANUP: pools: move the lock to the only __pool_get_first() that needs it

Now that __pool_alloc() only surrounds __pool_get_first() with the lock,
let's move it to the only variant that requires it and remove the ugly
ifdefs from the function. This is safe because nobody else calls this
function.