[Refactor][MoE] remove redundant code after refactoring fused_moe (#2612)

### What this PR does / why we need it?
There are a lot of redundant codes related to moe here, and the
structure is not very clear.
We did the following things：

we have placed the relatively independent code related to apply_mlp into
a separate file;
removed the environment variables of alltoall_buffer and alltoall_seq.
Remove the code related to alltoall_buffer and alltoall_seq, and retain
the sole TokenDispatcher inheritance class.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
e2e&ut

- vLLM version: v0.10.1.1
- vLLM main:
4071c76cf3

---------

Signed-off-by: Pr0Wh1teGivee <calvin_zhu0210@outlook.com>
Signed-off-by: weijinqian_v1 <weijinqian@huawei.com>
Co-authored-by: weijinqian0 <12153182+weijinqian0@users.noreply.github.com>

vllm_ascend/ascend_forward_context.py

@@ -35,10 +35,6 @@ def _get_fused_moe_state(ep_size: int, with_prefill: bool,
             return FusedMoEState.NaiveMulticast
         else:
             return FusedMoEState.AllGather
-    elif envs_ascend.VLLM_ASCEND_ENABLE_MOE_ALL2ALL_SEQ:
-        # MC2 Dispatch/Combine performs better than alltoall_seq in decoding stage.
-        return (FusedMoEState.All2AllSeq if
-                (ep_size < 16 or with_prefill) else FusedMoEState.MC2)
     # NOTE: mc2 need ep_size >= 16 & all2all can't use in torchair graph.
     elif ep_size < 16 or with_prefill:
         return FusedMoEState.All2All