Remove shortcode from instant search results

### Impacted plugin

Search

### What

If shortcodes cannot be expanded, then at least they should be removed from the results text

### How

It looks weird to have text like this [this_is_a_shortcode] in the middle of the results.

Reported in this forum issue: https://wordpress.org/support/topic/customize-results-pages-with-hooks/

Start in the Search plugin's instant-results code and trace how result text is prepared when shortcodes cannot be expanded. Reproduce the forum-reported case, then verify that markers such as [this_is_a_shortcode] no longer appear in instant results while surrounding text remains.

Written by the indexing model from the issue text.