Remove workaround which causes focus to return to select2

The workaround below causes a problem in the following use case:
1. open the drop down
2. click on the page (outside of the select2 control)
result: the focus returns to the select2 control

The issue that was supposed to be fixed by this workaround -
https://github.com/ivaynberg/select2/issues/939
An issue about the problem this workaround creates -
https://github.com/ivaynberg/select2/pull/1667