[gui] Remove deprecated labeling properties tab for new projects or older ones that don't use them

- Deprecated tab remains active for older projects, if any layer uses them
- Deprecated tab an be disabled/enabled via console commands for project entry, e.g.:
  QgsProject.instance().writeEntry('DeprecatedLabels', '/Enabled', bool) or
  QgsProject.instance().removeEntry('DeprecatedLabels', '/')