You could just remove the bullets by css, like .k2ItemsBlock ul li {list-style:none;}

If the modification you did was made in the core file of the module make sure to take a backup when you update K2 because it will be lost. It is better when you choose to customise anything to create an override first.
Read here: getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates how overrides work.