domain_conf: drop unused ref-count in snapshot object

domain_conf: drop unused ref-count in snapshot object

The ref count was assigned to 1 at creation, then never modified again
until it was decremented just before freeing the object.

* src/conf/domain_conf.h (_virDomainSnapshotObj): Delete unused
field.
(virDomainSnapshotObjUnref): Delete unused prototype.
* src/libvirt_private.syms: Likewise.
* src/conf/domain_conf.c (virDomainSnapshotObjNew)
(virDomainSnapshotObjListDataFree): Update users.
(virDomainSnapshotObjUnref): Delete.