Remove deprecated size_t overloads from RigActiveCellInfo

Migrate all callers to use type-safe ReservoirCellIndex and
ActiveCellIndex wrappers, then remove the deprecated isActive(size_t),
cellResultIndex(size_t), and setCellResultIndex(size_t, size_t) methods.

ApplicationLibCode/UnitTests/RigGridExportAdapter-Test.cpp

@@ -216,7 +216,7 @@ TEST( RigGridExportAdapterTest, CellActivity )
                 size_t origJ          = min.y() + j;
                 size_t origK          = min.z() + k;
                 size_t mainIndex      = mainGrid->cellIndexFromIJK( origI, origJ, origK );
-                bool   expectedActive = activeCellInfo->isActive( mainIndex );
+                bool   expectedActive = activeCellInfo->isActive( ReservoirCellIndex( mainIndex ) );
 
                 EXPECT_EQ( expectedActive, adapterActive ) << "Activity mismatch at (" << i << "," << j << "," << k << ")";
             }