Remove redundant code from Qwen3VLProcessor
Author: Xqle
Pull request: yes

# What does this PR do?
As per title. 

There are two identical lines assigning `video_grid_thw = videos_inputs["video_grid_thw"]` in `Qwen3VLProcessor.__call__()`.  One of them can be safely removed for cleaner and more concise code.
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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: qwen3_vl

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