MAINTAINERS: Drop outdated FPGA Manager website

The rocketboards website no longer really reflects a good landing
place for people interested in FPGA Manager.

Cc: Xu Yilun <yilun.xu@intel.com>
Cc: Wu Hao <hao.wu@intel.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>