Remove post type filter added by the plugin in the final query

The best course of action would be to prevent the plugin from doing something you don’t want. If for some reason this is not possible you can use the filter pre_get_posts to change the query before it is executed, like this:

add_action( 'pre_get_posts', 'wpse239465_force_post_type', 9999);

function wpse239465_force_post_type () {
  $query->set('post_type', 'mycustomepost');
  }

Note that this will force the post type on all your queries, given that the priority of 9999 guarantees it is the last filter applied and there is no condition attached to $query->set. So you may at least want to add some conditional tag.