Clean up and remove includes/media.variables.inc

I also want to get rid of media_variable_set() and media_variable_del() as those only do namespacing. For the default variables and media_variable_get() let's do the following:

 media.module:

/**
 * Internal default variables for media_variable_get().
 */
function media_variables() {
  return array(
    'media__wysiwyg_title' => t('Media browser'),
    'media__wysiwyg_icon_title' => t('Add media'),
    'media__wysiwyg_default_view_mode' => 'media_large',
    ...
  );
}

/**
 * Wrapper for variable_get() with automatic media namespacing and defaults.
 *
 * @param string $name
 *   The name of the media variable to return. Should not include the 'media__'
 *   prefix.
 * @param mixed $default_override
 *   (optional) The default value to pass along to variable_get(). If not
 *   provided, the default from media_variables() will be used if available.
 *   Otherwise, NULL will be used as the default.
 *
 * @return mixed
 *    The value of the variable from variable_get().
 */
function media_variable_get($name, $default_override = NULL) {
  $defaults = &drupal_static(__FUNCTION__);
  if (!isset($defaults)) {
    $defaults = media_variables();
  }

  // Add the media variable namespace.
  $name = 'media__' . $name;

  // Use the default variable from media_variables() if desired and it exists.
  $default = !isset($default_override) && isset($defaults[$name]) ? $defaults['name'] : $default_override;

  // Return the variable.
  return variable_get($name, $default);
}

Let's put this as a beta blocker. I do not want to add a dependency on the variable module. If it is worth doing optional integration, I would consider that, but I just want us to use normal variable_get() calls.