remove KernelApp.parent_appname

This allowed the Kernel to load config from ipython_qtconsole_config.py or ipython_notebook_config.py, depending on the parent that started the kernel. This is of limited usefulness, and can add weird, unexpected side effects.

We may not actually want to remove this feature, but it's just another bit of config complexity that I think we don't need, so I thought it was worth proposing.