Switch back to old check for reading pane for ol16

* src/ribbon-callbacks.cpp (get_mail_from_control): Use the
same codepath as OL2010 and 13 for 2016 again, too.

The crash in Outlook 2016 started to appear at some point.
Now after various updates to Outlook I can no longer reproduce it.
As the previewpane.wordeditor is not reliable. see:

We disable that codepath again. The crash report was

• GnuPG-Bug-Id: T3484