Update PYTHONPATH for main.py in document

[oatawa1]

What this patch does to fix the issue.

Some documents under blueoil/docs/usage directory refer to obsolete PYTHONPATH.
This PR update the PYTHONPATH.

Link to any relevant issues or pull requests.

Closes #1046

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[iizukak]

@oatawa1 In my understand, We do not need PYTHONPATH anymore.

[oatawa1]

@iizukak
I agree I think we don't need it when we install blueoil via pip -e .[cpu] or pip -e .[gpu],

But if we manually install the package via pip, we may need it. Precisely only PYTHONPATH=.
If not it will throw this error.

Traceback (most recent call last):
  File "blueoil/cmd/main.py", line 21, in <module>
    from blueoil.cmd.convert import convert as run_convert
ModuleNotFoundError: No module named 'blueoil'

My solution will be update the document about installation because we currently don't have it and remove PYTHONPATH from this PR.

[iizukak]

OA

[iizukak]

/ready

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[oatawa1]

@iizukak Thank you.