build: remove unused @edx/reactifex package

[feanil]

Remove @edx/reactifex package from devDependencies as it is no longer
needed. Translation extraction functionality has been verified to work
correctly without these dependencies.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.20%. Comparing base (be51d33) to head (aff0ff8).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   90.20%   90.20%           
=======================================
  Files         143      143           
  Lines        2206     2206           
  Branches      476      476           
=======================================
  Hits         1990     1990           
- Misses        204      208    +4     
+ Partials       12        8    -4     

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