build: Drop the @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 this dependency. We don't need it in the template
application.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (d0d7954) to head (36bad11).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #929   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           2       2           
  Lines           7       7           
======================================
  Misses          7       7           

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