build: remove unused @edx/reactifex package

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

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

package.json

@@ -34,7 +34,6 @@
     "@edx/brand": "npm:@openedx/brand-openedx@^1.2.2",
     "@edx/browserslist-config": "^1.1.1",
     "@edx/frontend-platform": "^8.3.1",
-    "@edx/reactifex": "^2.1.1",
     "@openedx/frontend-build": "^14.3.2",
     "@openedx/paragon": "^23.0.0",
     "@testing-library/dom": "^10.4.0",