Don't set the matplotlib backend

I've been becoming increasingly frustrated with the setting of the backend to Agg. I'm finding that in almost all situations I use this, it incorrectly sets the non-interactive backend and then I can't look at plots. I'm finding myself writing at least as much code in my scripts to avoid this behaviour than I would have to if plotting was breaking.

I would really like to see this removed.

I know that this is a convenient feature to have, especially for our examples, but it really isn't appropriate to have baked into a package which now has a number of downstream uses/applications. We could for example set this in the various bilby_pipe executables if necessary.

Thoughts? @gregory.ashton @matthew-pitkin I think you were involved in setting this up.

The fix would be to just remove https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/core/utils.py#L969-996