Tests: Remove some useless assertions from the image editor tests.

In each of these instances the variable being tested is a directly instantiated object and can never be a `WP_Error`, unlike when `wp_get_image_editor()` is used.

See #51344


git-svn-id: https://develop.svn.wordpress.org/trunk@49488 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/image/editorGd.php

@@ -531,7 +531,6 @@ class Tests_Image_Editor_GD extends WP_Image_UnitTestCase {
 		$expected = imagecolorsforindex( $image, $rgb );
 
 		$editor = new WP_Image_Editor_GD( $file );
-				$this->assertNotWPError( $editor );
 				$editor->load();
 				$editor->rotate( 180 );
 				$save_to_file = tempnam( get_temp_dir(), '' ) . '.png';