SQL/QSqlIndex: remove unused private function createField()

This function was forgotten when the Qt3 support was removed during
initial Qt5 porting (f306d18fe6555a9a5c60560745773b0bf5685ec3).

Change-Id: I83a0be0db7e81bd9d4f84683a013e1d4faab2dd8
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
(cherry picked from commit 5a7d41667d08cd6062cee43ee871fae5663ed7e3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/sql/kernel/qsqlindex.cpp

@@ -147,24 +147,6 @@ void QSqlIndex::setDescending(int i, bool desc)
         sorts[i] = desc;
 }
 
-/*! \internal
-
-  Creates a string representing the field number \a i using prefix \a
-  prefix. If \a verbose is true, ASC or DESC is included in the field
-  description if the field is sorted in ASCending or DESCending order.
-*/
-
-QString QSqlIndex::createField(int i, const QString& prefix, bool verbose) const
-{
-    QString f;
-    if (!prefix.isEmpty())
-        f += prefix + u'.';
-    f += field(i).name();
-    if (verbose)
-        f += u' ' + QString((isDescending(i) ? "DESC"_L1 : "ASC"_L1));
-    return f;
-}
-
 /*!
     \fn QString QSqlIndex::cursorName() const
 

src/sql/kernel/qsqlindex.h

@@ -41,7 +41,6 @@ public:
     void setDescending(int i, bool desc);
 
 private:
-    QString createField(int i, const QString& prefix, bool verbose) const;
     // ### Qt7: move to d-ptr
     QString cursor;
     QString nm;