Clean up temporary Extension class workarounds

Hm. The magically overloaded theme extension properties still appear to be used:

Undefined property: Drupal\Core\Extension\Extension::$info

Drupal\Core\Extension\ThemeHandler->addTheme(Object)
Drupal\Core\Extension\ThemeHandler->listInfo()
list_themes()
_drupal_maintenance_theme()
drupal_maintenance_theme()
drupal_flush_all_caches()

The reason is:

#2228093: Modernize theme initialization only cleaned up theme initialization. However, ThemeHandler::rebuildThemeData() is unrelated to initialization and still operates on Extension objects only.

Given that there is a use-case for instantiating a Theme domain object (including base themes) within that process, we should probably rename the ActiveTheme class into just Theme. — No change in behavior. It only doesn't have to be currently active theme, you can instantiate it for any theme.

However, the good news is that Drush has indeed been updated on testbots, so the other public property workarounds in the Extension class are no longer necessary. :-)

In turn, let's defer that theme data rebuilding clean-up to a separate issue.

Created #2326407: Remove magic property overloading from Extension class for cleaning up the serialization methods + refactoring the affected theme system functionality.

FWIW, this cleanup should not require a change notice.

The Extension class was only introduced in D8. The workarounds removed here only had to be added for Drush compatibility (on testbots). The Extension class has proper methods to retrieve all values of interest already.

No code was supposed to use the temporarily added public properties. All of them were annotated with crystal clear @todos.