[ENH]: Support SubFigure.remove()

[anntzer]

Problem

Currently SubFigures cannot be remove()d from the parent figure. The API would be more consistent if that was possible.

Proposed solution

Implement SubFigure.remove().

[fernandaitoda]

Hi, can you explain with details what this new feature would be able to do? maybe a sample code to show what were you trying to do

[jklymak]

sfig.remove() would remove the subfigure and all its children from the figure. Note that our removal machinery is non-trivial, but doing something similar as what is done for axes should work. I'm not 100% sure how the tree gets traversed.

If someone works on this, it would be nice to write up your notes so we can document how remove is meant to work.