Doc: removed reference to obsolete classes

QSocket and QSocketDevice are not part of Qt 5.4

Task-number: QTBUG-40362
Change-Id: Ieffd992c203af94cac0eb21a630b6ac98754f358
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>

src/corelib/io/qdatastream.cpp

@@ -261,12 +261,6 @@ QDataStream::QDataStream()
 /*!
     Constructs a data stream that uses the I/O device \a d.
 
-    \warning If you use QSocket or QSocketDevice as the I/O device \a d
-    for reading data, you must make sure that enough data is available
-    on the socket for the operation to successfully proceed;
-    QDataStream does not have any means to handle or recover from
-    short-reads.
-
     \sa setDevice(), device()
 */