A declaration of war is only used when fighting a sovereign entity. The peace treaty that follows the war rescinds the declaration of war.
When fighting a non-sovereign, a congressional declaration authorizing the use of force is the traditional tool, going all the way back to Jefferson and the Barbary Pirates. As a rule, when hostilities cease, the declaration is left on the books. This is simply a matter of sloppy bookkeeping, and nothing further should be read into it.
What do you make of the Senate vote to cut off Aid to Saudi in their fight against Yemen derpsters.
Does it have any real world effect, or is it just for show?
Bagster