Revert "Mark toString() method as deprecated, scheduled for removal in 4.0.0"

This reverts commit 4c467ce4d5.

For more information, please see the discussion at:
https://github.com/ramsey/uuid/commit/4c467ce4d5a72c3cf0832c813d4d84d222c3d4bb#commitcomment-31174263

Also refer to the discussion at:
https://github.com/ShittySoft/symfony-live-berlin-2018-doctrine-tutorial/pull/3#issuecomment-460441229

TL;DR: I am retaining `toString()` for improved static analysis and
long-term stability.

src/UuidInterface.php

@@ -268,10 +268,6 @@ interface UuidInterface extends JsonSerializable, Serializable
     /**
      * Converts this UUID into a string representation.
      *
-     * @deprecated In ramsey/uuid 4.0.0, this method will be replaced with the
-     *     __toString() magic method, which is currently available in the
-     *     Uuid concrete class. The new recommendation is to cast Uuid objects
-     *     to string, rather than calling `toString()`.
      * @return string
      */
     public function toString();