How to delete read more span on single post view?

As stated in the docs, if the quicktag <!--more--> is used in a post to designate the “cut-off” point for the post to be excerpted, the_content() tag will only show the excerpt up to the <!--more--> quicktag point on non-single/non-permalink post pages.

WordPress adds <span id="more-' . $post->ID . '"></span> to the content in this case. Since there’s no content in the <span> (check out the code), you can easily remove the <span> by filtering the content:

add_filter( 'the_content', function( $content ) {
  return str_replace( '<span id="more-' . get_the_ID() . '"></span>', '', $content );
}

Note that this causes the Read More link not to jump to the area where you inserted <!--more--> anymore. An easier way to achieve this would be to use the the_content_more_link filter and remove the #more-<post_id> part from it there. There are a few examples in the Codex for that.