PwBaseWorkChain validation of resources is incorrect

[sphuber]

It assumes that num_machines and max_wallclock_seconds are set, however, this only applies to certain schedulers. For example for the SgeScheduler you cannot specify num_machines so the workchain is fundamentally incompatible with those schedulers. The solution is simple: just remove the validator. The reason is that this validator was added at a time when the resources for the scheduler were not yet validated in aiida-core. But now they are (should check starting from what version) so now we can just rely on this, and even better, the validation will happen on instantiation and not when the workchain starts running.