Doc: Only list qt core classes in qt core io group

The group page for Qt core classes related to I/O include classes from
other modules. Remove these classes from the group; it may confuse
the reader.

Fixes: QTBUG-110020
Change-Id: If7df85523ce6b3aa09605bd89d9899ce308d2671
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
(cherry picked from commit 76ecff6aebb35f4e39439aab8d18d828c3c8a1fd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/src/io.qdoc

@@ -10,7 +10,7 @@
     network handling.
 
     These \l{Qt Core} classes are used to handle input and output to and from
-    external devices, processes, files etc. as well as manipulating files and
+    external devices, processes, files etc., as well as manipulating files and
     directories.
 
 */

src/gui/image/qimagereader.cpp

@@ -11,7 +11,6 @@
     \inmodule QtGui
     \reentrant
     \ingroup painting
-    \ingroup io
 
     The most common way to read images is through QImage and QPixmap's
     constructors, or by calling QImage::load() and

src/gui/image/qimagewriter.cpp

@@ -9,7 +9,6 @@
     \inmodule QtGui
     \reentrant
     \ingroup painting
-    \ingroup io
 
     QImageWriter supports setting format specific options, such as
     compression level and quality, prior to storing the

src/gui/text/qtextdocumentwriter.cpp

@@ -41,7 +41,6 @@ public:
     \inmodule QtGui
 
     \ingroup richtext-processing
-    \ingroup io
 
     To write a document, construct a QTextDocumentWriter object with either a
     file name or a device object, and specify the document format to be