Remove unused code for converters directory path validation

[Irozuku]

Summary

Removes redundant absolute path calculations and existence checks, the setup process is more streamlined and less prone to path-related failures (executable).

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

Briefly list the important modified files and what was done.

• DashAI/back/job/converter_job.py: Removed the logic for calculating and validating the converters directory path in instantiate_converters, reducing complexity and potential points of failure.

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Testing (optional)

• Verify that the converters are still correctly instantiated and that the removal of the explicit path check does not interfere with the job execution environment.
• Verify that converters are working properly in the executable version of the app.