[Model] Remove redundant softmax when using PoolingType.STEP

[Maybewuss]

Whether to enable softmax can be controlled by the softmax parameter when initializing Pooler.

https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/layers/pooler.py#L143

The softmax here is unnecessary.

[github-actions]

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[DarkLight1337]

Thanks for fixing!