Remove dependency on Prepopulate

Cannot install using composer:

composer require 'drupal/on_page_help:^1.0'
./composer.json has been updated
Running composer update drupal/on_page_help

Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires drupal/on_page_help ^1.0 -> satisfiable by drupal/on_page_help[1.0.0].
    - drupal/on_page_help 1.0.0 requires composer/prepopulate ^2.4 -> could not be found in any version, there may be a typo in the package name.

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.
 - It's a private package and you forgot to add a custom repository to find it

Read <https://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.

Installation failed, reverting ./composer.json and ./composer.lock to their original content.

@maxilein: fixed in 1.0.1. Try it again, please.

The dependency has been removed earlier. Just moving it to needs review