timedata: Prevent warning overkill

The "please check your computer's data and time" message when the clock
deviates from the network currently generates an overkill of messages:

    orion@lethe:~/bitcoin$ src/bitcoind
    Warning: Warning: Please check that your computer's date and time are correct! If your clock is wrong Bitcoin Core will not work properly.

In the log:

    2015-09-27 16:24:13 *** Warning: Please check that your computer's date and time are correct! If your clock is wrong Bitcoin Core will not work properly.
    2015-09-27 16:24:13 Warning: Warning: Please check that your computer's date and time are correct! If your clock is wrong Bitcoin Core will not work properly.

Remove one level of 'Warning:' and reduce to one log message.

src/timedata.cpp

@@ -99,9 +99,8 @@ void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
                 if (!fMatch)
                 {
                     fDone = true;
-                    string strMessage = _("Warning: Please check that your computer's date and time are correct! If your clock is wrong Bitcoin Core will not work properly.");
+                    string strMessage = _("Please check that your computer's date and time are correct! If your clock is wrong Bitcoin Core will not work properly.");
                     strMiscWarning = strMessage;
-                    LogPrintf("*** %s\n", strMessage);
                     uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
                 }
             }