Remove Proxy Symbol Workaround for Flexible Sync

[takameyer]

What, How & Why?

According to the workaround comments, the symbol injection for detecting proxy collections in flexible sync is no longer needed. I have removed the workaround and successfully tested it in the example application. This PR reverts the changes from #4541.

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 📝 Update COMPATIBILITY.md
• 🚦 Tests
• 🔀 Executed flexible sync tests locally if modifying flexible sync
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated

[kraenhansen]

If there's no harm in keeping it in, I'd rather that we don't touch master unless we really need to backport some bugfix. This feels like a simplification/ enhancement which does have some potential to break stuff if it doesn't work on some ancient version of an os / configuration.

[takameyer]

@kraenhansen It's currently blocking me. Without this change, the jest tests won't allow me to console.log a collection. It's baffling, but I really don't want to dig deeper. After cherry-picking this change into my current work, things are working fine.

[kraenhansen]

Sorry for the fast approval, I had a deeper look at the proposed changes.
Are the deletion of lib/mutable-subscription-set.js intentional? With that we can't release a v11?

[takameyer]

Are the deletion of lib/mutable-subscription-set.js intentional?

Yes, it was added only for this symbol hack. See https://github.com/realm/realm-js/pull/4541/files