src: remove unused DSAKeyExportJob

[panva]

Removes the unused DSAKeyExportJob, a remnant of Node-only Web Cryptography algorithms.

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (91dadf2) to head (8c05d0e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #59291      +/-   ##
==========================================
- Coverage   90.00%   90.00%   -0.01%     
==========================================
  Files         649      649              
  Lines      192180   192160      -20     
  Branches    37659    37652       -7     
==========================================
- Hits       172970   172946      -24     
+ Misses      11830    11808      -22     
- Partials     7380     7406      +26     

Files with missing lines	Coverage Δ
src/crypto/crypto_dsa.cc	79.16% <ø> (+17.62%)	⬆️
src/crypto/crypto_dsa.h	0.00% <ø> (ø)

... and 28 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68328/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68330/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68331/