[python-package] drop support for h2o datatable

[jameslamb]

Closes #6662

This proposes fully dropping support for datatable.DataTable input in the Python package.

Notes for Reviewers

Benefits of this change

• reduced maintenance burden
• reduced code complexity
• helps with effort to add type hints ([python-package] type hints in python package #3756 and [ci] warnings from mypy #3867)

See #6662 for more details.

Why now?

In #6662 I'd originally proposed keeping the support in the package with a deprecation warning for "2-3" releases, and it's only been 1 so far (#6670)... but I think it's safe to make this change. Some evidence:

• no user has commented [RFC] [python-package] remove h2o datatable support? #6662 and nothing else has been linked to it
• xgboost has fully dropped support for this type of input: [breaking] [py] Drop support for datatable. dmlc/xgboost#11070

And specifically, I just observed a case where keeping this around led to extra complexity and effort for a contributor: #6892 (comment)

How I searched for stuff to remove

git grep -i h2o
git grep -i datatable

[borchero]

Nice, love to see some reduction in complexity 😁