Remove dummy dataset writing from RecordComponent::flush()

[franzpoeschel]

See #489 and #512 for a description. The exactly same thing happens also in RecordComponent.cpp. PR #512 did not catch this.

[ax3l]

CI seams unhappy.
You might want to implement the default for such records and attributes on Series::close() or similar.

[franzpoeschel]

The problem is with the fileBased_write_empty_test which triggers exactly the situation that I have described here:

It is a design decision in ADIOS (also ADIOS2) not to physically create variables (used for openPMD datasets) unless data is actually written and openPMD can leverage this behavior. The workaround removed in this PR tries to circumvent this behavior. Needs to be documented in this case.

Relevant part of the test code:

    Dataset dset = Dataset(Datatype::DOUBLE, {2});
    {
        Series o = Series("../samples/subdir/serial_fileBased_write%T." + backend, AccessType::CREATE);

        ParticleSpecies& e_1 = o.iterations[1].particles["e"];
        e_1["position"][RecordComponent::SCALAR].resetDataset(dset);
        e_1["positionOffset"][RecordComponent::SCALAR].resetDataset(dset);
        o.iterations[1].setTime(1.f);
        ParticleSpecies& e_2 = o.iterations[2].particles["e"];
        e_2["position"][RecordComponent::SCALAR].resetDataset(dset);
        e_2["positionOffset"][RecordComponent::SCALAR].resetDataset(dset);
        o.iterations[2].setTime(2.f);
        ParticleSpecies& e_3 = o.iterations[3].particles["e"];
        e_3["position"][RecordComponent::SCALAR].resetDataset(dset);
        e_3["positionOffset"][RecordComponent::SCALAR].resetDataset(dset);
        o.iterations[3].setTime(3.f);
    }

Since no data is ever written, the ADIOS variable is not created in storage and not read back in.

Implementing a default as you suggest would be equivalent to writing dummy data, just at a later point.
If we go for this, we should keep #516 in mind. Should we?