Remove differential view support from some Issues widgets

Indeed, this support does not work correctly. For instance, for "False Positive Issues", the diff. period dates are used to search on the creation date of the issues whereas it should be on the issue changelog (= when the issue status changed to "false-positive"). The consequence is that in diff mode, the widget does not show what the user expects to see.

So here the widgets for which diff support should be removed:

• False Positive Issues ("false_positive_issues.html.erb")
• My Unresolved Issues ("my_unresolved_issues.html.erb")