Remove guava

[i1619khz]

I searched for the use of guava in the jcasbin code and found only in used in Util.java. Only used for md5 encryption

https://github.com/casbin/jcasbin/blob/a21b2199bcd1199a66aebf7e86930b2218dfacd1/src/main/java/org/casbin/jcasbin/util/Util.java#L45

https://github.com/casbin/jcasbin/blob/a21b2199bcd1199a66aebf7e86930b2218dfacd1/src/main/java/org/casbin/jcasbin/util/Util.java#L292-L295

Referring to the practice of common codec, only need to jdk

  public static MessageDigest getDigest(String algorithm) {
      try {
          return MessageDigest.getInstance(algorithm);
      } catch (NoSuchAlgorithmException e) {
          throw new IllegalArgumentException(e);
      }
  }

  public static String md5(String data) {
      return new String(getDigest("MD5").digest(data.getBytes(StandardCharsets.UTF_8)));
  }

Reduce unnecessary dependencies

[casbin-bot]

@tangyang9464 @seriouszyx @elfisworking @fangzhengjin

[tangyang9464]

@i1619khz Could you make a PR？

[i1619khz]

@i1619khz Could you make a PR？ with pleasure

[i1619khz]

@tangyang9464 #280