Remove --zstd option (always using zstd)

[kelson42]

Fixes #1642

[kelson42]

@kelvinhammond BTW you can also remove the ability to compress using LZMA in openzim/libzim as well. See openzim/node-libzim#79

[codecov]

Codecov Report

Merging #1643 (38ee8b7) into master (2448e2b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1643      +/-   ##
==========================================
- Coverage   69.51%   69.50%   -0.02%     
==========================================
  Files          26       26              
  Lines        2401     2400       -1     
  Branches      469      468       -1     
==========================================
- Hits         1669     1668       -1     
  Misses        568      568              
  Partials      164      164              

Impacted Files	Coverage Δ
src/mwoffliner.lib.ts	66.54% <ø> (+0.23%)	⬆️
src/parameterList.ts	100.00% <ø> (ø)
src/Downloader.ts	73.11% <0.00%> (-0.33%)	⬇️

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[kelvinhammond]

I'll look at this, this evening.

[kelson42]

@kelvinhammond Got any time to review? May I merge this small PR?