KVM: arm64: Remove unused return value from kvm_reset_vcpu()

Get rid of the return value for kvm_reset_vcpu() as there are no longer
any cases where it returns a nonzero value.

Link: https://lore.kernel.org/r/20230920195036.1169791-8-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>