Remove special flattening-behavior for Wizard content providers

[ptziegler]

This removes the feature from the "New" and "Import/Export" wizards, where container for wizard elements would be flattened, if it only contains a single, other container.

There are two reasons for this change:

• it is not consistent with the other viewers, that always show the complete tree path (e.g. the preference dialog).
• it doesn't work properly, as it is still possible to produce the undesirable behavior in the "New" wizard (for e.g. Java/JUnit/...)

See #2602 for context.

[ptziegler]

This is an example of the flattening, where the intermediate Test Import Parented Category entry is skipped, making it look like Test Import Add To Parent Category is a direct child of General.

This is an example using the same filter, but in the New wizard. By the same logic, the JUnit entry should be skipped, even though it isn't...

[github-actions]

Test Results

 1 214 files  +  607   1 214 suites  +607   1h 30m 13s ⏱️ + 59m 8s
 7 729 tests ±    0   7 496 ✅ +    1  233 💤  -   1  0 ❌ ±0 
16 232 runs  +8 116  15 717 ✅ +7 835  515 💤 +281  0 ❌ ±0 

Results for commit 5d0d4fa. ± Comparison against base commit b97ea88.

[vogella]

Linux build failure seems unrelated, see #2603