Drupal.theme is not a constructor, don't use Drupal.theme.prototype

I just realized that Drupal.theme is never used as a constructor to make an object but is always used as a typical object.

Drupal.theme will explicitly look for a default implementation in the prototype object, for all intent and purpose we could be using mickey instead of prototype.

And better, just remove the extra object altogether and just implement Drupal.theme.myThemeFunction. I'm not aware of a module needing to override a JS theme function and still access the default implementation.

// Before
Drupal.theme.prototype.myThemeFunction = function () {};

// After
Drupal.theme.myThemeFunction = function () {};