Removed redundant assertIsString

Unless the return type of of Gravity class' validate function is changed, this will always be a string.

Author: JeffAlyanak

Tests/GravityTest.php

@@ -20,7 +20,6 @@ public function testGravity($gravity): void
 
         $validatedGravity = $gravity->validate();
 
-        static::assertIsString($validatedGravity);
         static::assertNotEmpty($validatedGravity);
 
         $command = new Command(IMAGEMAGICK_DIR);