Remove weight public property from theme extensions

Problem/Motivation

#2659940: Extension System, Part III: ThemeExtensionList and ThemeEngineExtensionList introduced a weight public property on Themes' extension objects. There was no discussion and we still have comments like

// The value is not used; the weight is ignored for themes currently.

and runtime code like

      // Set the actual theme weights.
      $theme_list = array_map(function ($theme) use ($theme_data) {
        return $theme_data[$theme]->sort;
      }, $theme_list);

      // Sort the theme list by their weights (reverse).
      arsort($theme_list);
      $theme_list = array_keys($theme_list);

Proposed resolution

Remove the weight property because it is confusing and unnecessary.

This need to be done before 8.7.0 otherwise it is anAPI change.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

N/a