fileOutput.param: How to disable particles

This pull request:
- removes `fileOutput.unitless`: moved to fileOutput.param since #457
- Adds a small comment on how to disable particle output.

**Misc:**

Added a note in the [wiki](https://github.com/ComputationalRadiationPhysics/picongpu/wiki/Plugin%3A-HDF5) that `fileOutput.param` relates to HDF5.

Adios is controlled by the same file I guess?
- [ ] update [adios page](https://github.com/ComputationalRadiationPhysics/picongpu/wiki/Plugin%3A-ADIOS), too