riscv: mm: Remove setup_zero_page()

The empty_zero_page sits at .bss..page_aligned section, so will be
cleared to zero during clearing bss, we don't need to clear it again.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>