RISC-V: Remove mem_end command line processing

This is just some cruft left over from before the port converted to
device tree.  The right way to handle memory regions is to specify them
in the device tree, which BBL (our simplest bootloade) is already
capable of doing.  This patch simply removes the cruft.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>