QDate: remove an old comment

QDateTime::addMsecs does check for overflow. I don't know when this
 has changed, but it doesn't matter.

Change-Id: I44c6ba5e88cce544c0d1ef33fa38a528a96b0b7e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/time/qdate/tst_qdate.cpp

@@ -652,7 +652,6 @@ void tst_QDate::startOfDay_endOfDay_bounds()
     QVERIFY(last.isValid());
     QVERIFY(first < epoch);
     QVERIFY(last > epoch);
-    // QDateTime's addMSecs doesn't check against {und,ov}erflow ...
     QVERIFY(!first.addMSecs(-1).isValid() || first.addMSecs(-1) > first);
     QVERIFY(!last.addMSecs(1).isValid() || last.addMSecs(1) < last);