[ROCm] Remove unnecessary assertion of max_model_len in ROCM_AITER_MLA attention backend.

[vllmellm]

This PR removes the unnecessary constraint and assertion of a specific max_model_len value from the ROCM_AITER_MLA attention backend on both VLLM v1 and v0 engines.

lm_eval results on DeepSeek-V2-Lite-Chat with default engine args on both VLLM v0 and v1 engines.

VLLM_USE_V1=1 VLLM_ROCM_USE_AITER=1 VLLM_ROCM_USE_AITER_MOE=0 VLLM_ROCM_USE_AITER_RMSNORM=0 VLLM_ROCM_USE_AITER_LINEAR=0 lm_eval --model vllm --model_args pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=1,block_size=1 --trust_remote_code --tasks gsm8k --num_fewshot 5 --batch_size auto

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.6657	±	0.0130
		strict-match	5	exact_match	↑	0.6543	±	0.0131

VLLM_USE_V1=0 VLLM_ROCM_USE_AITER=1 VLLM_ROCM_USE_AITER_MOE=0 VLLM_ROCM_USE_AITER_RMSNORM=0 VLLM_ROCM_USE_AITER_LINEAR=0 lm_eval --model vllm --model_args pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=1,block_size=1 --trust_remote_code --tasks gsm8k --num_fewshot 5 --batch_size auto

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.5974	±	0.0135
		strict-match	5	exact_match	↑	0.5876	±	0.0136

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