[Rector] Apply Rector: RemoveUnusedPrivatePropertyRector

[samsonasik]

Remove unused private properties with RemoveUnusedPrivatePropertyRector rector rule.

Checklist:

• Securely signed commits

[samsonasik]

Temporary add symplify/composer-json-manipulator to require-dev due error in https://github.com/codeigniter4/CodeIgniter4/runs/2556009947#step:10:5 :

Run vendor/bin/rector process --dry-run --no-progress-bar

Error: ] Class                                                                  
         "Symplify\ComposerJsonManipulator\Bundle\ComposerJsonManipulatorBundle"
         not found   

ref rectorphp/rector#6405 .