Col delete func

[mathemancer]

Fixes #3583

Adds a columns.delete function to the RPC endpoint.

Technical details

The function is documented in its docstring and the Mathesar docs, and said documentation should be sufficient.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

@Anish9901 Should be ready to review.

[Anish9901]

The PR looks good to me @mathemancer, please fix the name of the test that I've pointed down below and merge this.

[Anish9901]

This function name doesn't make sense.