[ntuple] Remove redundant LoadEntry call

The entry is currently loaded twice: once on `RNTupleProcessor::Advance`,
which is called when the iterator is incremented, and once when the
iterator is dereferenced. This second call is unnecessary and only
causes the processor to take more time, so this commit removes it.

tree/ntuple/v7/inc/ROOT/RNTupleProcessor.hxx

@@ -220,11 +220,7 @@ public:
          return obj;
       }
 
-      reference operator*()
-      {
-         fProcessor.LoadEntry();
-         return *fProcessor.fEntry;
-      }
+      reference operator*() { return *fProcessor.fEntry; }
 
       friend bool operator!=(const iterator &lh, const iterator &rh)
       {