Cleanup to prepare for using mmap pointer in external memory. (#9317)

- Update SparseDMatrix comment.
- Use a pointer in the bitfield. We will replace the `std::vector<bool>` in `ColumnMatrix` with bitfield.
- Clean up the page source. The timer is removed as it's inaccurate once we swap the mmap pointer into the page.

tests/cpp/tree/gpu_hist/test_evaluate_splits.cu

@@ -285,8 +285,6 @@ TEST(GpuHist, PartitionTwoNodes) {
                                     dh::ToSpan(feature_histogram_b)};
     thrust::device_vector<GPUExpandEntry> results(2);
     evaluator.EvaluateSplits({0, 1}, 1, dh::ToSpan(inputs), shared_inputs, dh::ToSpan(results));
-    GPUExpandEntry result_a = results[0];
-    GPUExpandEntry result_b = results[1];
     EXPECT_EQ(std::bitset<32>(evaluator.GetHostNodeCats(0)[0]),
               std::bitset<32>("10000000000000000000000000000000"));
     EXPECT_EQ(std::bitset<32>(evaluator.GetHostNodeCats(1)[0]),