remove t5 legacy code

[dimapihtar]

What does this PR do ?

Removes T5 legacy model as a part of MLM code cleanup.

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[dimapihtar]

/ok to test e6ab124

[dimapihtar]

/ok to test f1074b0

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24336560611

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24342208972

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24343095079