To delete issues, simply tap and hold an issue in the issues list and then tap the bin icon in the editing toolbar that appears. You can select multiple issues once in edit mode by tapping each one as normal, as well as selecting them all via the option at the top. You can also clear your selection via the overflow menu.
Once deleted, an option to Undo will appear at the bottom of the screen. This will only be available for a few seconds - after that, those issues are permanently deleted.