Do not automatically resolve default Matplotlib backend

[ianthomas23]

There is an issue reported in Matplotlib (matplotlib/matplotlib#28855) in which use of a config file ~/.ipython/profile_default/ipython_kernel_config.py containing c.InteractiveShellApp.matplotlib = 'inline' results in the default Matplotlib backend being enabled rather than matplotlib-inline. I think this has occurred historically in some situations (such as #25) but is now more likely given the recent movement of the backend-handling code from IPython to Matplotlib.

My analysis (matplotlib/matplotlib#28855 (comment)) is that it is a simple fix in matplotlib-inline to allow us to check what the current Matplotlib backend is without automatically enabling the defaults backend if none has yet been specified. I will submit a PR with this fix shortly, when I have performed some comprehensive manual testing that it does not break some other current use case.