Remove dead codes.

[czp13]

Description of the bug

I believe there are some opportunities to remove unused codes.

Similar to this PR (removed dead codes which I have observed accidentally)

• fix, chore: Removing some dead code (now is the perfect time: selenium 4.9.0. is more strict on being w3c) #16665

This dead code was blocking to update to the newest 4.9.0 selenium (and to any new selenium versions after this).
Tests can be a low-hanging fruit, but maybe there are some other utils or other classes which are easier to remove dead code.

Short analysis executed shown 562 opportunities:

Probably not all of them can be removed as some of them are part of abstract classes which used elsewhere, but these methods for example could be removed:

GwtEventHandlerTest.assertPolymerMethods

JsonUtilsTest
.createNumberStreamForNull()
.createObjectStreamForNull()
.createStreamForNull()

And so on...

Expected behavior

The repository could contain fewer unused code lines, which makes:

• dependency addition, removal, bumping,
• refactoring, changes also easier,
• and just to overview understand the logic, code base better as code is simplified a bit.
• potentially builds/compilation time can be faster if we remove a lot of dead codes, although by not much probably :D

Minimal reproducible example

In IntelliJ:
Analyze | Run Inspection by Name... | Unused declaration.
(Potentially worth to add Test regexp or Util regexp, as there will be more false positives otherwise)

Versions

• Vaadin / Flow version: latest

[czp13]

Only private methods, and more investigation is needed before removing methods.