[Refactor][EAGLE] 3/N delete redundant methods in mtp_proposer (#5420)

### What this PR does / why we need it?
This PR aims to delete redundant methods in mtp_proposer. All the
deleted methods now can be found in eagle_proposer. We also remove some
methods in eagle_proposer since they are identical to those in
vllm-eagle.

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested?
by ci

- vLLM version: release/v0.13.0
- vLLM main:
81786c8774

---------

Signed-off-by: Zetong Li <slippersss@126.com>

tests/ut/spec_decode/test_mtp_proposer.py

@@ -262,6 +262,7 @@ class TestMtpProposer:
                                             device=torch.device("cpu"))
         assert torch.equal(next_token_ids, expected_next_tokens)
 
+    @patch("vllm_ascend.spec_decode.eagle_proposer.HAS_TRITON", False)
     @patch("vllm.v1.spec_decode.eagle.CpuGpuBuffer")
     def test_prepare_inputs_padded(self, mock_cpu_gpu_buffer):
         mock_buffer_instance = MagicMock()