remove legacy Bert code

[dimapihtar]

What does this PR do ?

Removes legacy Bert code, classification.py & multiple_choice.py as a part of MLM code cleanup.

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[dimapihtar]

/ok to test 58a123f

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24336909593

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24339637376