os: drop snapid_t arg to collection_list

os: drop snapid_t arg to collection_list

All this does is filter out objects with old snapids, but this can be
just as easily done in the one caller that specifies a non-zero value
(PGLS and PGNLS), which in fact already do that filtering.

Signed-off-by: Sage Weil <sage@redhat.com>