Remove the sizes attribute from thumbnail images in Twenty Nineteen

The AMP runtime sets an inline style on an <amp-img> based
on the sizes attribute if it's present.
For example, <amp-img style="width:calc(50vw)">.
Removing the 'sizes' attribute isn't ideal, but it looks
like it's not possible to override that inline style.
This is only a workaround until the issue is addressed
in the amphtml repo.

includes/sanitizers/class-amp-core-theme-sanitizer.php

@@ -53,18 +53,20 @@ class AMP_Core_Theme_Sanitizer extends AMP_Base_Sanitizer {
 	protected static $theme_features = array(
 		// Twenty Nineteen.
 		'twentynineteen'  => array(
-			'dequeue_scripts'                    => array(
+			'dequeue_scripts'                             => array(
 				'twentynineteen-skip-link-focus-fix', // This is part of AMP. See <https://github.com/ampproject/amphtml/issues/18671>.
 				'twentynineteen-priority-menu',
 				'twentynineteen-touch-navigation', // @todo There could be an AMP implementation of this, similar to what is implemented on ampproject.org.
 			),
-			'remove_actions'                     => array(
+			'remove_actions'                              => array(
 				'wp_print_footer_scripts' => array(
 					'twentynineteen_skip_link_focus_fix', // See <https://github.com/WordPress/twentynineteen/pull/47>.
 				),
 			),
-			'add_twentynineteen_masthead_styles' => array(),
-			'add_twentynineteen_image_styles'    => array(),
+			'add_twentynineteen_masthead_styles'          => array(),
+			'add_twentynineteen_image_styles'             => array(),
+			'remove_twentynineteen_thumbnail_image_sizes' => array(),
+
 		),
 
 		// Twenty Seventeen.
@@ -342,6 +344,26 @@ public static function set_twentyseventeen_quotes_icon() {
 		} );
 	}
 
+	/**
+	 * Remove the sizes attribute from thumbnail images in Twenty Nineteen.
+	 *
+	 * The AMP runtime sets an inline style on an <amp-img> based on the sizes attribute if it's present.
+	 * For example, <amp-img style="width:calc(50vw)">.
+	 * Removing the 'sizes' attribute isn't ideal, but it looks like it's not possible to override that inline style.
+	 *
+	 * @todo: remove when this is resolved: https://github.com/ampproject/amphtml/issues/17053
+	 * @since 1.0
+	 */
+	public static function remove_twentynineteen_thumbnail_image_sizes() {
+		add_filter( 'wp_get_attachment_image_attributes', function( $attr ) {
+			if ( isset( $attr['class'] ) && false !== strpos( $attr['class'], 'attachment-post-thumbnail' ) ) {
+				unset( $attr['sizes'] );
+			}
+
+			return $attr;
+		}, 11 );
+	}
+
 	/**
 	 * Add filter to adjust the attachment image attributes to ensure attachment pages have a consistent <amp-img> rendering.
 	 *