[Dev][Cherry-Pick Main] Remove legacy grouped gemm and grouped mlp

[yaox12]

What does this PR do ?

Some changes from #3410 and #3770 are missing in main to dev sync. So cherry-pick them manually.

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[yaox12]

/ok to test 7943d55

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25838005381

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25841588519