[fix] remove mixtral from is_fa3_default_architecture (sgl-project#6191)

Author: Alcanderian

python/sglang/srt/utils.py

@@ -2076,7 +2076,6 @@ def is_fa3_default_architecture(hf_config):
         "Llama4ForConditionalGeneration",
         "LlamaForCausalLM",
         "MistralForCausalLM",
-        "MixtralForCausalLM",
         "Gemma2ForCausalLM",
         "Gemma3ForConditionalGeneration",
         "Qwen3ForCausalLM",