Remove BC layer from \Drupal\Core\DrupalKernel::getInstallProfile()

Problem/Motivation

#2156401: Write install_profile value to configuration and only to settings.php if it is writeable added a BC layer to read the install profile from settings.php instead of configuration. This can be removed in 9.x.

Proposed resolution

Remove

    else {
      // If system_update_8300() has not yet run fallback to using settings.
      $install_profile = Settings::get('install_profile');
    }

And simplify code.

Remaining tasks

User interface changes

API changes

None

Data model changes

None