RISC-V: Remove unused class definitions

RISC-V: Remove unused class definitions

Removes a whole lot of unnecessary boilerplate code. Machines
don't need to be objects. The expansion of the SOC object model
for the RISC-V machines will happen in the future as SiFive
plans to add their FE310 and FU540 SOCs to QEMU. However, it
seems that this present boilerplate is complete unnecessary.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>