minor #15198 [HttpFoundation] Remove unused var (Dave Hulbert)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #15198).

Discussion
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[HttpFoundation] Remove unused var

Minor. Looks like the var `$content` got introduced in aa5e6165, then in 8982c324 the early return meant it wasn't needed.

| Q             | A
| ------------- | ---
| Fixed tickets | N/A
| License       | MIT

Commits
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da5218f Remove var not used due to returning early (introduced in 8982c32)

src/Symfony/Component/HttpFoundation/Request.php

@@ -476,7 +476,6 @@ class Request
      */
     public function __toString()
     {
-        $content = '';
         try {
             $content = $this->getContent();
         } catch (\LogicException $e) {