Add a remove method to ExtraCoords for removing a coordinate

[DanRyanIrish]

Description

[kc611]

Hey @DanRyanIrish , I'd like to make a PR for this.

From what I gather, here we need a .remove() method that will simply remove a tuple from the the self._lookup_tables, which corresponds to the coordinates specified by the user (originally added over here):

https://github.com/sunpy/ndcube/blob/947799ea213728d6215cb791508ef186ca53b24f/ndcube/extra_coords/extra_coords.py#L206

My doubt here is, is .remove() expected to take the user input similar to .add() ? (name, array_dimension and lookup_table). If so then we can just replicate what the .add() method does and remove the finally generated tuple from the list.

Or is there something else .remove() should do that I'm missing over here ?