minor #14366 [HttpFoundation] remove getExtension method (dosten)

This PR was squashed before being merged into the 2.7 branch (closes #14366).

Discussion
----------

[HttpFoundation] remove getExtension method

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

Since the minimum PHP version in 2.7 is 5.3.9 we can delete the `getExtension` method and use the real `\SplFileInfo::getExtension()`

Commits
-------

55ecf3b [HttpFoundation] remove getExtension method

src/Symfony/Component/HttpFoundation/File/File.php

@@ -87,20 +87,6 @@ class File extends \SplFileInfo
         return $guesser->guess($this->getPathname());
     }
 
-    /**
-     * Returns the extension of the file.
-     *
-     * \SplFileInfo::getExtension() is not available before PHP 5.3.6
-     *
-     * @return string The extension
-     *
-     * @api
-     */
-    public function getExtension()
-    {
-        return pathinfo($this->getBasename(), PATHINFO_EXTENSION);
-    }
-
     /**
      * Moves the file to a new location.
      *

src/Symfony/Component/HttpFoundation/Tests/File/FileTest.php

@@ -148,12 +148,6 @@ class FileTest extends \PHPUnit_Framework_TestCase
         @rmdir($targetDir);
     }
 
-    public function testGetExtension()
-    {
-        $file = new File(__DIR__.'/Fixtures/test.gif');
-        $this->assertEquals('gif', $file->getExtension());
-    }
-
     protected function createMockGuesser($path, $mimeType)
     {
         $guesser = $this->getMock('Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface');