remove LegacyApiTestCase from code

[krasilnikovs]

In this commit, https://github.com/symfony/maker-bundle/commit/4da6e4a9615c8a32bae48630626ca7dac2af06a0 was removed import related to LegacyApiTestCase but MakerTest class still use LegacyApiTestCase::class as a result generated API test case class with incorrect import.

use Symfony\Bundle\MakerBundle\Maker\LegacyApiTestCase;