Deprecate detection of IPython frontends

The check for the Qt console won't be necessary in IPython 3 and above: we decided that the Qt console's display of HTML reprs was so bad that it shouldn't attempt to show them. We also want to remove the `parent_appname` config value (ipython/ipython#4980), which will break both of these functions (making them always return False).

The assignment to `ipnbh` was redundant - nothing used that variable. I guess it was used previously and didn't get cleaned up during some changes (quite possibly my own omission).