minor #17431 [Serializer] Remove useless is_object condition (paradajozsef)
minor #17431 [Serializer] Remove useless is_object condition (paradajozsef)

This PR was merged into the 2.7 branch.

Discussion
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[Serializer] Remove useless is_object condition

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

[This condition](https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Serializer/Serializer.php#L185) is useless, because ```$class``` is not used in ```getNormalizer()```.

Commits
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f6fd5e4 Remove useless is_object condition

symfony/serializer,symfony/Serializer