RJS-2870: Remove an unused header from the iOS and unused import from Android binding

[kraenhansen]

What, How & Why?

This closes #6808 by removing an import which is no longer used.
The fix was verified by the original creator of the issue here: #6808 (comment)

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 📝 Update COMPATIBILITY.md
• 🚦 Tests (by the original reporter)
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 💥 Breaking label has been applied or is not necessary
• 🔔 Mention @realm/devdocs if documentation changes are needed

[sunrise1002]

So in this file
node_modules/realm/binding/android/src/main/java/io/realm/react/RealmReactModule.java
we also have to remove this line for android 🤔
import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags;