Remove dead code from Container::reset()

Problem/Motivation

While investigating another issue I spotted this code in \Drupal\Component\DependencyInjection\Container::reset():

    if (!empty($this->scopedServices)) {
      throw new LogicException('Resetting the container is not allowed when a scope is active.');
    }

Scoped services were deprecated in Symfony 2.8 and removed in Symfony 3: https://symfony.com/blog/new-in-symfony-2-8-deprecating-scopes-and-intro...

We removed most mentions of scoped services in #2712647: Update Symfony components to ~3.2 but this one was missed.

Steps to reproduce

Proposed resolution

Remove these lines.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet