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.