Revert catboost note about GPU backend

[nzw0301]

What is an issue?

The catboost callback does not work with GPUs as noted in optuna/optuna#3903. However the last stable catboost has supported the callback on GPUs (ref), so the note can be removed.

[nzw0301]

I realised that python package does not support custom callback with GPU at https://github.com/catboost/catboost/blob/master/catboost/python-package/catboost/_catboost.pyx#L1940.