The result of TimestampWritableV2.toString() is wrong when year is larger than 9999.

Hi, I found that `TimestampWritableV2.toString()` can be wrong when year is larger than 9999. Here is what I get:

• test code:

  // code placeholder
  @Test
  public void testTimestampWritableV2toString() {
    TimestampWritableV2 timestampWritableV2 = new TimestampWritableV2(
        Timestamp.valueOf("10001-01-01 01:01:23"));
    
    assertEquals("+10001-01-01 01:01:23", timestampWritableV2.toString());
  }

• output:

// code placeholder
org.junit.ComparisonFailure:
 Expected :+10001-01-01 01:01:23
 Actual :+10001-01-01 01:01:2323

The patch below removes some 'wrong' code in `TimestampWritableV2.toString()`, for the reason that the length of `org.apache.hadoop.hive.common.type.Timestamp#toString` can be larger than 19, even its nano-of-second is zero.