mem: Remove DRAMSim2 DDR3 configuration

mem: Remove DRAMSim2 DDR3 configuration

This patch prunes the DDR3 config that was initially created to match
the default config of DRAMSim2. The config is not complete as it is,
and to avoid having to maintain it, the easiest way forward is to
simply prune it. Going forward we are adding power number etc to the
other configurations.