Issue #864: Remove widget subclasses that used filter_the_content().

That function has been removed in PR #888.
It looks like that uses full-page buffering.
So remove the widget subclasses that use that.
@todo: look at regressions in the 'Archives' and 'Categories' dropdowns.

includes/class-amp-autoloader.php

@@ -81,12 +81,7 @@ class AMP_Autoloader {
 		'AMP_WP_Utils'                                => 'includes/utils/class-amp-wp-utils',
 		'AMP_Widget_Archives'                         => 'includes/widgets/class-amp-widget-archives',
 		'AMP_Widget_Categories'                       => 'includes/widgets/class-amp-widget-categories',
-		'AMP_Widget_Media_Audio'                      => 'includes/widgets/class-amp-widget-media-audio',
-		'AMP_Widget_Media_Gallery'                    => 'includes/widgets/class-amp-widget-media-gallery',
-		'AMP_Widget_Media_Image'                      => 'includes/widgets/class-amp-widget-media-image',
-		'AMP_Widget_Media_Video'                      => 'includes/widgets/class-amp-widget-media-video',
 		'AMP_Widget_Recent_Comments'                  => 'includes/widgets/class-amp-widget-recent-comments',
-		'AMP_Widget_RSS'                              => 'includes/widgets/class-amp-widget-rss',
 		'WPCOM_AMP_Polldaddy_Embed'                   => 'wpcom/class-amp-polldaddy-embed',
 		'AMP_Test_Stub_Sanitizer'                     => 'tests/stubs',
 		'AMP_Test_World_Sanitizer'                    => 'tests/stubs',