Undo slapd/add.c rev 1.54; back-bdb2 and back-ldbm were the only backends

Undo slapd/add.c rev 1.54; back-bdb2 and back-ldbm were the only backends
that called entry_free() on failure. This change would cause a memory leak
from all the other backends. Instead, remove the entry_free calls in these
two backends and let the frontend take care of it for everyone.