[python-package] remove unused functions in xgboost.data

[jameslamb]

Proposes removing two unused internal functions in the Python package:

• xgboost.data._check_complex()
• xgboost.data._to_data_type()

I'm unsure when the uses of these functions were removed, but I'm pretty confident that they're no longer used.

grep -R '_check_complex'

[trivialfis]

Thank you again for cleaning up the code! Let me double-check the _check_complex, it was required by sklearn estimator tests, I'm not sure why it's passing now.

[jameslamb]

Ok yeah no problem!

[trivialfis]

Just checked, the sklearn estimator check parametrize_with_checks no longer passes complex data for validation.