Cleanup: remove ReverseGeoLookupThread

Fetching the taxonomy from GPS coordinates was implemented in
a QThread. But the only access to the main function was a
direct call to run(). Thus, the thread was *never* started.
The function call was always asynchronous [it was using an
event loop though, so the UI doesn't hang]. Notably this
means that the signals connected to the thread would never
fire. And the spinner would never be activated.

Thus:
1) Turn the thread into a simple function.
2) Remove the spinner.

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

core/divesitehelpers.h

@@ -2,18 +2,6 @@
 #ifndef DIVESITEHELPERS_H
 #define DIVESITEHELPERS_H
 
-#include "units.h"
-#include <QThread>
-
-class ReverseGeoLookupThread : public QThread {
-Q_OBJECT
-public:
-	static ReverseGeoLookupThread *instance();
-	void lookup(struct dive_site *ds);
-	void run() override;
-
-private:
-	ReverseGeoLookupThread(QObject *parent = 0);
-};
+void reverseGeoLookup();
 
 #endif // DIVESITEHELPERS_H