Remove theme_system_powered_by()

I'm struggling to understand why this is even a theme function, actually. If you wanted to customize this wouldn't you just make your own block, or even string override for that matter?

Adding a commit message to the issue summary so the folks who already worked on #1987410: [meta] system.module - Convert theme_ functions to Twig and #1898454: system.module - Convert PHPTemplate templates to Twig (comment #41 and above aka @c4rl) get credit.

Well the only use of this theme function is in the powered by block:

http://api.drupal.org/api/search/8/SystemPoweredByBlock

So it seems like we could just move the code there.

(New patch so no interdiff.)

So this theme function was originally added in #183332: Add an optional 'Powered by Drupal' block.

While I like the following thought from @eaton, I'm not sure any themes out there actually do this (a survey of the 25 most popular themes from https://drupal.org/project/project_theme says nope!).

An interesting side note. This means that theme developers interested in promoting themselves with a "Designed by so-and-so" blurb can stick that stuff in an overridden theme_system_powered_by(); It would never show up for folks who've already decided they don't want the promotional block.

So I'm not seeing the real use case for providing the ability to override the output of this block via a theme function.

Markup before:

<div class="block block-system contextual-region" id="block-bartik-powered" role="complementary">
  
    <div data-contextual-id="block:block=bartik_powered:"></div>

  <div class="content">
    <span>Powered by <a href="http://drupal.org">Drupal</a></span>
  </div>
</div>

Markup after (it's identical):

<div class="block block-system contextual-region" id="block-bartik-powered" role="complementary">
  
    <div data-contextual-id="block:block=bartik_powered:"></div>

  <div class="content">
    <span>Powered by <a href="http://drupal.org">Drupal</a></span>
  </div>
</div>

Updating the summary to reflect the proposed patch.

Thanks! Change notice:
https://drupal.org/node/2153753