riscv: remove free_initrd_mem

The RISC-V free_initrd_mem is identical to the default one, except
that it doesn't poison the freed memory.  Remove it so that the
default implementations gets used instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>