PDF gnerators can't be managed by the service container

Problem/Motivation

When resolving mpdf from the service container you'll get the following error

Mpdf/MpdfException with message 'Cannot write to an undeclared property Mpdf/Mpdf::$_serviceId'

Steps to reproduce

Drupal::service('mpdf');

Proposed resolution

Remove it from services.yml

Remaining tasks

Going to create a PR

User interface changes

None

API changes

There shouldn't be any API changes since 99.9% of users would be instantiating mpdf themselves with their config. This is what this module is doing in MpdfGenerator too, so should be fine to remove it.

Data model changes

None.