fix register mappings pass to only register mappings with one object manager

| Q             | A
| ------------- | ---
| Bug fix?      |  yes
| New feature?  | no
| BC breaks?    | yes (but feature is totally new)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7599
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/2507

This PR fixes the issues @Stof discovered in #7599 - we now introduce a way to only register the mappings with one of the object managers.