fix(cdk/tree): retainining previous objects

[naaajii]

fixes component retaining old data in memory even when its not used causing memory usage to increase

fixes #30322

[mmalerba]

It looks like there might be a bug with this. There's an internal test failing because some nested child nodes have the same aria-level as their parent while the parent is collapsed. They're display: none while collapsed, so its probably not an issue from an accessibility perspective, but it causes treeHarness.getNodes({level: parentLevel}) to also pick up the hidden children