merge-ort: remove compile-time ability to turn off usage of memory pools

Simplify code maintenance by removing the ability to toggle between
usage of memory pools and direct allocations.  This allows us to also
remove paths_to_free since it was solely about bookkeeping to make sure
we freed the necessary paths, and allows us to remove some auxiliary
functions.

Suggested-by: Jeff King <[email protected]>
Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren