simplify event listener removal

This is not a big deal, just a little code cleanup. It seems you handled the bug from the $.proxy functions (which can't be used for event removal) [here](https://github.com/dangrossman/bootstrap-daterangepicker/commit/e3d1d06c355a3cc493c7498e6345d232ac7aa55c). You don't have any other event listeners on the document, so you can just use the namespace to remove them.