[sgl-kernel Code Clean] Remove useless lightning_attention kernel (sgl-project#13819)

Author: BBuf

benchmark/kernels/minmax-text-01-lightning_attention/benchmark_lightning_attention_decode.py

@@ -269,7 +269,6 @@ set(SOURCES
     "csrc/allreduce/mscclpp_allreduce.cu"
     "csrc/attention/cascade.cu"
     "csrc/attention/cutlass_mla_kernel.cu"
-    "csrc/attention/lightning_attention_decode_kernel.cu"
     "csrc/attention/merge_attn_states.cu"
     "csrc/attention/vertical_slash_index.cu"
     "csrc/common_extension.cc"