No longer rely on the DOM to deselect last selection

When choosing an option in a single select, we need to update the
array of options to deselect any previously selected options.
result_single_selected is no longer used
Move some repeated logic into abstract chosen
Make sure selected items are cleared after resetting Chosen single
Remove superfluous variable update
Merge pull request #1461 from harvesthq/g1458-fix-single-selected-result

Single select results in sync