remove misleading statement from ~QSqlDatabase()

The presence of the sentence "If this is the last QSqlDatabase object
that uses a certain database connection, the database connection is
automatically closed" is misleading, whether or not the statement is
true. It is about an internal detail of QSqlDatabase's implementation.
As such it is not appropriate for user documentation. The user should be
focused on how to use addDatabase(), cloneDatabase() and removeDatabase()
correctly.

Task-number: QTBUG-29481
Change-Id: I0c39584be260e13340834c34098368fcce4a7419
Reviewed-by: Florian Paul Schmidt <mista.tapas@gmx.net>
Reviewed-by: Sune Vuorela <sune@vuorela.dk>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/kernel/qsqldatabase.cpp

@@ -794,9 +794,6 @@ void QSqlDatabasePrivate::init(const QString &type)
 /*!
     Destroys the object and frees any allocated resources.
 
-    If this is the last QSqlDatabase object that uses a certain
-    database connection, the database connection is automatically closed.
-
     \sa close()
 */