riscv/vmm: Remove a redundant maxcpu check in vm_alloc_vcpu()

[markj]

Summary

aplic_max_cpu_count() just returns the VM's max vCPU count, and
vm_alloc_vcpu() already checks that.

Just remove this check, so that it's easier to merge vm_alloc_vcpu()
into MI code.

If the APLIC really does require us to lower the limit, we should
instead adjust vm->maxcpu in vm_create().

[br]

looks redundant. thanks