Related Posts: Remove the call to remove_filter( 'the_content'… (#13804)

Fixes #13775

The filter 'filter_add_target_to_dom()' removes itself from the 'the_content'
hook. Then, a bug in Core causes the filter with the next highest priority to
be skipped.

For more information on the Core bug see: https://core.trac.wordpress.org/ticket/9968

To avoid this bug, remove the call to "remove_filter( 'the_content', array( $this,
'filter_add_target_to_dom'), ...)". Instead, just rely on conditionals in the
'filter_add_target_to_dom()' method to control when related posts are added to the
content.

Related posts should not be added to the content under these three condtions:
1. The post contains a Related Posts block.
2. The post contains a 'jetpack-related-posts' shortcode.
3. The 'get_the_excerpt' hook is being executed. This hook is executed when the related
   posts are being generated, and related posts should not be added to the related post
   content.

These conditions are already handled by the 'filter_add_target_to_dom()' method. However,
legacy related posts are currently added to posts that contain a Related Posts block. Fix this
by removing the $content parameter from "has_block('jetpack/related-posts', $content)".

Author: kbrown9

modules/related-posts/jetpack-related-posts.php

@@ -163,7 +163,8 @@ public function get_headline() {
 
 	/**
 	 * Adds a target to the post content to load related posts into if a shortcode for it did not already exist.
-	 * Will skip adding the target if the post content contains a Related Posts block.
+	 * Will skip adding the target if the post content contains a Related Posts block or if the 'get_the_excerpt'
+	 * hook is in the current filter list.
 	 *
 	 * @filter the_content
 	 *
@@ -172,7 +173,7 @@ public function get_headline() {
 	 * @returns string
 	 */
 	public function filter_add_target_to_dom( $content ) {
-		if ( has_block( 'jetpack/related-posts', $content ) ) {
+		if ( has_block( 'jetpack/related-posts' ) ) {
 			return $content;
 		}
 
@@ -369,17 +370,7 @@ public function render_block( $attributes ) {
 			'size'            => ! empty( $attributes['postsToShow'] ) ? absint( $attributes['postsToShow'] ) : 3,
 		);
 
-		$excludes      = $this->parse_numeric_get_arg( 'relatedposts_origin' );
-
-		$target_to_dom_priority = has_filter(
-			'the_content',
-			array( $this, 'filter_add_target_to_dom' )
-		);
-		remove_filter(
-			'the_content',
-			array( $this, 'filter_add_target_to_dom' ),
-			$target_to_dom_priority
-		);
+		$excludes = $this->parse_numeric_get_arg( 'relatedposts_origin' );
 
 		$related_posts = $this->get_for_post_id(
 			get_the_ID(),
@@ -452,7 +443,7 @@ public function render_block( $attributes ) {
 	 *
 	 * @uses absint
 	 *
-	 * @param string $arg Name of the GET variable
+	 * @param string $arg Name of the GET variable.
 	 * @return array $result Parsed value(s)
 	 */
 	public function parse_numeric_get_arg( $arg ) {