Remove mdfreader's PyQt dependency
Remove mdfreader's PyQt dependency

* [mdfreader](https://github.com/ratal/mdfreader) comes with two modules: the main module, also named `mdfreader`, and a console application, `mdfconverter`.
* `mdfconverter` is optional and requires PyQt to run.
* `mdfconverter has been removed from the conda package.
* Therefore it is ok to remove `pyqt` from the list of conda dependencies for `mdfreader`.
goanpeca/staged-recipes,ocefpaf/staged-recipes,jakirkham/staged-recipes,kwilcox/staged-recipes,ocefpaf/staged-recipes,stuertz/staged-recipes,ReimarBauer/staged-recipes,hadim/staged-recipes,goanpeca/staged-recipes,mariusvniekerk/staged-recipes,johanneskoester/staged-recipes,kwilcox/staged-recipes,conda-forge/staged-recipes,johanneskoester/staged-recipes,conda-forge/staged-recipes,stuertz/staged-recipes,hadim/staged-recipes,jochym/staged-recipes,jakirkham/staged-recipes,jochym/staged-recipes,ReimarBauer/staged-recipes,mariusvniekerk/staged-recipes