reset delete-flag

Since 'PathFormat' objects are being reused, setting `delete`
to True once caused all files downloaded after to be deleted as well.

gallery_dl/util.py

@@ -474,6 +474,7 @@ class PathFormat():
     def finalize(self):
         """Move tempfile to its target location"""
         if self.delete:
+            self.delete = False
             os.unlink(self.temppath)
             return