riscv: canaan: remove useless select of non-existing config SYSCON

The config SYSCON never existed in the kernel repository; so, the select of
that config in ./drivers/soc/canaan/Kconfig has no effect.

Presumably, this was just some mistake, assuming some symmetry in handling
and naming of configs that simply does not exist.

Remove this useless select of a non-existing config.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>