MainTab::refreshDisplayedDiveSite() was used after dive-site edit
to update the information of the location entry box. This should
always display the dive-site of the currently shown/edited dive,
therefore it makes no sense to use the displayed_dive_site here.
Simply use the dive site of displayed_dive.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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