Playground still keeps reference to bound objects after removing bindings

[Rinzwind]

Scenario:

• Open a new playground and do object := MockObject new
• Open the bindings for the playground, press the button to remove them, then close the bindings window
• Open another new playground and inspect Smalltalk garbageCollect. MockObject allInstances size
• Close the first playground and inspect the snippet in the other one again

At the last step the result is 0, but at the step before the result is 1, implying the playground unexpectedly still has a reference to the object despite the bindings having been removed.

Tested using: Pharo 13 build 262.