Remove unused mlc_llm.op.position_embedding module (#3424)

This was a stale duplicate of
tvm.relax.frontend.nn.llm.position_embedding.
Verified removal is safe by:
Searching for all imports from mlc_llm.op.position_embedding (none
found)
Testing that mlc_llm.op imports correctly after cleanup

Author: babusid

python/mlc_llm/op/__init__.py

@@ -6,5 +6,4 @@
 from .extern import configure, enable, get_store
 from .ft_gemm import faster_transformer_dequantize_gemm
 from .pipeline_parallel import pipeline_stage_boundary
-from .position_embedding import llama_rope
 from .top_p_pivot import top_p_pivot, top_p_renorm