docs: remove design section in docs

There is actually no valid information in the 'design' section, so just
remove this part from the project document.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Liangxing Wang <Liangxing.Wang@arm.com>
Reviewed-by: Yefeng Lin <Yefeng.Lin@arm.com>
Change-Id: If56bdc524361f60e6aabe34d77e8082034e14bc1