ManiphestReportController (which renders global /maniphest/report/burn/) wastes CPU cycles. Thus remove useless code.

It constructs table rows never to be shown: The code calculates rows and adds them via $table = new AphrontTableView($rows), then defines $panel = new PHUIObjectBoxView() and does $panel->setTable($table) but a bit later $panel = id(new PhabricatorProjectBurndownChartEngine()) overwrites that table and all its data.
This re-definition of $panel was introduced in rP5c1b91ab457db9f3db10d8cc5e07831512645ebb.

Its code creates a PhabricatorProjectBurndownChartEngine anyway, similar to PhabricatorProjectReportsController. So none of those expensive database queries are needed at all. Variables like list($burn_x, $burn_y) = $this->buildSeries($data); are never read since rPf8ebc71b8f217ed156f416ddb4cd028dcaa28174.

Bug: T16005

1. Make sure /applications/view/PhabricatorFactApplication/ is installed
2. Go to /maniphest/report/burn/ for global view, go to some URL like /maniphest/report/burn/?project=PHID-PROJ-1234567890abcdef for per-project view
3. See that the "Burnup Rate" chart is rendered in the same way before and after applying this patch
4. Optionally, run ./bin/cache purge --all (but seems not relevant here) and reload
5. Optionally, go to /project/reports/1/ and as expected the burndown chart is the same as in step 2