Always attach to "pre_get_posts" hook.

You can't use is_search when adding filters because that can only be used after the `posts_selection` hooks (according to [Conditional Tags in the Codex](https://codex.wordpress.org/Conditional_Tags)).

I'm really very new to WordPress and plug-ins, but I believe it does not hurt to just always hook to the `pre_get_posts` and remove posts anyway.