RISC-V: Remove duplicate command-line parsing logic

builtin_cmdline handling is present in drivers/of/fdt.c so the
duplicate logic in arch/riscv/setup.c results in duplication of
the builtin command line. e.g. CONFIG_CMDLINE="root=/dev/vda ro"
gets appended twice and gives "root=/dev/vda ro root=/dev/vda ro"

Before this patch:

[    0.000000] Kernel command line: root=/dev/vda ro root=/dev/vda ro

After this patch:

[    0.000000] Kernel command line: root=/dev/vda ro

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