commit-graph: remove Future Work section

The commit-graph feature began with a long list of planned benefits, most of which are now complete. The future work section has only a few items left. As for making more algorithms aware of generation numbers, some are only waiting for generation number v2 to ensure the performance matches the existing behavior using commit date. It is unlikely that we will ever send a commit-graph file as part of the protocol, since we would need to verify the data, and that is expensive. If we want to start trusting remote content, then that item can be investigated again. While there is more work to be done on the feature, having a section of the docs devoted to a TODO list is wasteful and hard to keep up-to-date. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>