Revert unintended effects of #1966

[tuncbkose]

Revert to the previous default filename behavior

$ ls
test.ipynb
$ jupyter nbconvert --to notebook --output out.ipynb test.ipynb 
[NbConvertApp] Converting notebook test.ipynb to notebook
[NbConvertApp] Writing 72 bytes to out.ipynb
$ jupyter nbconvert --to notebook test.ipynb 
[NbConvertApp] Converting notebook test.ipynb to notebook
[NbConvertApp] Writing 72 bytes to test.nbconvert.ipynb

Closes #1970.

Edit:
Additional things to fix:

• Cannot convert equally named notebooks in different directories in-place #1979 same nb names in different directories
  • Fixed by removing the incorrect check that was failing (and an associated test). As a result, if output_base doesn't include some format section (like {notebook_name}), some files might get overwritten. This should not happen unless the user specifically wants it.
• I don't think using {notebook_filename} will always work the way it was intended. Even if it does, it doesn't have any tests, which I should add.
  • Just removed this functionality as it doesn't seem to work cohesively with existing FilesWriter configuration.

Now also closes #1979.

[tuncbkose]

@meeseeksdev tag bug

Edit: Wasn't sure if I could label, guess not

[lumberbot-app]

Awww, sorry tuncbkose you do not seem to be allowed to do that, please ask a repository maintainer.

[tuncbkose]

Not sure if failures are related

[krassowski]

I restarted the failing CI jobs, maybe these are just flaky.

[tuncbkose]

Rebasing onto main

[tuncbkose]

Failure seems unrelated to me

[blink1073]

Thank you!