Delete broken ZstdCompressor constructor

[nhz2]

This method of the ZstdCompressor constructor is undocumented, untested, and appears not to be used downstream; therefore, this PR removes it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.35%. Comparing base (b9630e6) to head (8f9724e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   64.19%   64.35%   +0.16%     
==========================================
  Files           4        4              
  Lines         391      390       -1     
==========================================
  Hits          251      251              
+ Misses        140      139       -1     

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[mkitti]

This was the signature of the default constructor before I added endOp. I added it in case any code had used the default constructor directly.

[mkitti]

IMHO it might be slightly friendlier to deprecate the constructor first before removing it.

[nhz2]

It is hard to deprecate because, from what I can tell, there is no public way to get a cstream.

But I'll wait for a PR to add the deprecation warning, or for you to approve #83 before making the 0.8.7 release.

Also, I found this issue using JET.jl. Maybe we should add JET and ExplicitImports to CI somehow (though in my experience, they can have false positives, so I'm not sure how to handle that).