Remove useless code

Author: alexandre-daubois

ByteString.php

@@ -340,7 +340,7 @@ public function reverse(): static
     public function slice(int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
-        $str->string = (string) substr($this->string, $start, $length ?? \PHP_INT_MAX);
+        $str->string = substr($this->string, $start, $length ?? \PHP_INT_MAX);
 
         return $str;
     }