arch: riscv: pmp: fix return value of arch_mem_domain_partition_remove()

If no thread use this memory domain, there isn't any user PMP region
translated from memory partitions in domain. In this case, memory
partition removal doesn't need to remove user PMP region and
arch_mem_domain_partition_remove() could just successfully return.

Signed-off-by: Jim Shu <cwshu@andestech.com>