Remove kernel config from parent config files

those config files are no longer read by the kernel

- Allow `--help` and config_file class lists to differ (due to weird mixin behavior)
- Add `ipython_kernel_config.py`
- kernel config still shows up in `qtconsole --help`,
  but not in the `ipython_qtconsole_config.py`


- [x] Still need to check any docs that refer to putting kernel config in the parent app's config file.

closes #6533
Is there a use case for having both help_classes and config_classes different from classes, or did you just prefer to define it symmetrically?