[wpp] shortcode won't render the excerpt unless we explicitely add it to the HTML code via post_html

[cabrerahector]

Ever since its introduction the [wpp] shortcode will not render the excerpt even if the user sets its length via excerpt_length as instructed. This is confusing because the user doesn't know that they also need to add the {excerpt} Content Tag somewhere in their shortcode via post_html.

Let's review this part of the code so that:

• Using the post_html isn't necessary anymore to render the excerpt.
• Nothing breaks for already-in-use [wpp] shortcodes out there to avoid negatively impacting the layout(s) people are using out there with this shortcode.

[cabrerahector]

See: https://wordpress.org/support/topic/post-excerpt-not-displaying-2/