Cleanup: remove DiveItem::icon_names member array

Each DiveItem (which is a wrapper around diveId with some virtual
functions), had a member icon_names, which is an array of
four QStrings. These were not used anywhere and must be an obscure
oversight and was probably planned as a static cons array?.
In any case, remove it.

There *was* a function-local analogous icon_names array in
DiveItem::data() though. This array would initialize four
QStrings from C-string literals on every invocation. Make
this array static, local to the translation unit and use
the QStringLiteral macro to construct the QString object at
compile-time.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

qt-models/divetripmodel.h

@@ -47,7 +47,6 @@ public:
 	QString displayTags() const;
 	int countPhotos(dive *dive) const;
 	int weight() const;
-	QString icon_names[4];
 };
 
 struct TripItem : public TreeItem {