riscv: Cleanup setup_bootmem()

riscv: Cleanup setup_bootmem()

After the following patches,

  commit 2c80e632e314 ("RISC-V: Fix usage of memblock_enforce_memory_limit")
  commit 87e3be6fd226 ("RISC-V: Remove any memblock representing unusable memory area")
  commit 920c7b70a912 ("arch, drivers: replace for_each_membock() with for_each_mem_range()")

some logic is useless, kill the mem_start/start/end and unneeded code.

Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>