Remove useless 'format' setting when download thumbnails and subtitles

[benoit74]

Fix #351

Changes:

• do not pass the format option to yt_dlp when downloading thumbnails or subtitles, this makes the download fail
• add debug log when thumbnail or subtitle download succeeded

[benoit74]

This is not really the solution, just a random way to partially fix the real issue.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 1.51%. Comparing base (b0015c9) to head (e4a27a9).

Files with missing lines	Patch %	Lines
scraper/src/youtube2zim/scraper.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #354      +/-   ##
========================================
- Coverage   1.51%   1.51%   -0.01%     
========================================
  Files         11      11              
  Lines       1119    1123       +4     
  Branches     170     170              
========================================
  Hits          17      17              
- Misses      1102    1106       +4     

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