Remove "not yet implemented" from flush comments

[adinauer]

📜 Description

Sentry.flush as well as other flush methods had a comment saying "Not yet implemented" which is no longer true.

💡 Motivation and Context

Fixes #4304

💚 How did you test it?

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	432.11 ms	490.49 ms	58.38 ms
Size	1.58 MiB	2.22 MiB	655.16 KiB

Previous results on branch: fix/flush-comment-noop

Startup times

Revision	Plain	With Sentry	Diff
30f5b61	438.10 ms	500.38 ms	62.28 ms

App size

Revision	Plain	With Sentry	Diff
30f5b61	1.58 MiB	2.22 MiB	655.17 KiB