Revert gemma3n fast prefill changes

[sarckk]

Purpose

Multi-modal processor tests for H2OVLChatModel and KimiVLForConditionalGeneration are broken on trunk after merging #22628.

I was previously unable to reproduce this issue locally as it requires that these test cases come after the Gemma3n test case. The failures are:

H20VLChatModel

TypeError: _LazyConfigMapping.__init__() missing 1 required positional argument: 'mapping'

Kimi (OOM)

(EngineCore_0 pid=1298081)   File "/data/users/yhshin/gitrepos/vllm/vllm/model_executor/models/moonvit.py", line 141, in sdpa_attention
(EngineCore_0 pid=1298081)     attn_output = F.scaled_dot_product_attention(q,
(EngineCore_0 pid=1298081)                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_0 pid=1298081) torch.OutOfMemoryError: CUDA out of memory. Tried to allocate 254.47 GiB. GPU 0 has a total capacity of 94.99 GiB of which 78.18 GiB is free. Including non-PyTorch memory, this process has 16.80 GiB memory in use. Of the allocated memory 16.03 GiB is allocated by PyTorch, and 104.27 MiB is reserved by PyTorch but unallocated. If reserved but unallocated memory is large try setting PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True to avoid ...

This PR reverts gemma3n changes in #22628 (commit c498483) which fixes the tests.

Test Plan

pytest tests/models/multimodal/processing/test_tensor_schema.py::test_model_tensor_schema -s 

Test Result

Multi-Modal Processor Test should pass in CI

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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[gemini-code-assist]

Code Review

This PR reverts the gemma3n fast prefill feature to fix failing tests. The changes within the model files are correct. However, the revert is incomplete, leaving behind dead code, configuration options, and tests related to the removed feature. I've added a critical comment pointing out the need for a complete revert to maintain code health.

[heheda12345]

Need to ensure Multi-modal processor tests pass before merging this PR.

[heheda12345]

@sarckk v1-test-e2e-plus-engine fails and is related to this PR.