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>

arch/riscv/mm/init.c

@@ -86,11 +86,6 @@ static void __init setup_initrd(void)
 	initrd_start = 0;
 	initrd_end = 0;
 }
-
-void __init free_initrd_mem(unsigned long start, unsigned long end)
-{
-	free_reserved_area((void *)start, (void *)end, -1, "initrd");
-}
 #endif /* CONFIG_BLK_DEV_INITRD */
 
 void __init setup_bootmem(void)