[Misc] Remove VLLM_USE_V1 usage in code

[wangxiyuan]

We plan to remove V0 code from this version. The first step is to delete v0 usage.

Related: #1620

• vLLM version: v0.9.2
• vLLM main: vllm-project/vllm@61e2082

[codecov]

Codecov Report

Attention: Patch coverage is 81.48148% with 10 lines in your changes missing coverage. Please review.

Project coverage is 54.20%. Comparing base (c30ddb8) to head (e01ec7d).
Report is 126 commits behind head on main.

Files with missing lines	Patch %	Lines
vllm_ascend/models/deepseek_dbo.py	0.00%	5 Missing ⚠️
vllm_ascend/models/deepseek_v2.py	0.00%	4 Missing ⚠️
vllm_ascend/platform.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1764       +/-   ##
===========================================
+ Coverage   27.39%   54.20%   +26.81%     
===========================================
  Files          56       82       +26     
  Lines        6191     9995     +3804     
===========================================
+ Hits         1696     5418     +3722     
- Misses       4495     4577       +82     

Flag	Coverage Δ
unittests	54.20% <81.48%> (+26.81%)	⬆️

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