MAINTAINERS: Remove Jianguo Sun from HiSilicon STB DWC entry

Jianguo Sun's email address reported in MAINTAINERS is not active any
longer, remove it from the DWC HiSilicon STB entry.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>