Remove file `__toString` behaviour

Casting a file to a string to get the content is a bit of a dirty
pattern.

src/Node/File.php

@@ -103,12 +103,4 @@ class File implements FileInterface
     {
         return strlen($this->content);
     }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function __toString()
-    {
-        return $this->content;
-    }
 }

src/Node/FileInterface.php

@@ -20,9 +20,4 @@ interface FileInterface extends NodeInterface
      * @param string $content
      */
     public function setContent($content);
-
-    /**
-     * @return string The file content
-     */
-    public function __toString();
 }

test/unit/Node/FileTest.php

@@ -29,13 +29,6 @@ class FileTest extends UnitTestCase
         $this->assertEquals('foo', $file->getContent());
     }
 
-    public function testToString()
-    {
-        $file = new File('foo');
-
-        $this->assertEquals('foo', (string) $file);
-    }
-
     public function testGetDateAccessed()
     {
         $file = new File();