Global SheetsRegistry leaks memory?

Expected behavior: Created StyleSheet instances should not be retained indefinitely.

Describe the bug: StyleSheet instances are being retained inside the global SheetsRegistry.

Codesandbox link: Run the react-jss example code https://github.com/cssinjs/react-todomvc-jss.git

1. Take memory snapshot.
2. Add a few items.
3. Delete those items.
4. Repeat step 2 and 3 a few more times.
5. Take another memory snapshot.
6. Filter both snapshots by constructor named StyleSheet and you'll find the number only grows.

Versions (please complete the following information):

• jss: The example is using 6.5.0, but I believe the issue is present in all versions.
• Browser [e.g. chrome, safari]: Chrome
• OS [e.g. Windows, macOS]: MacOS Feel free to add any additional versions which you may think are relevant to the bug.