Implement NestedFrame.__delitem__ for nested columns

[hombit]

Feature request

It would be nice if we can do del nf['nested.x'], currently it raises KeyError. I believe we should implement __delitem__ for that

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.