Doc: Fix issues in print media type in offline style

When generating HTML output, QDoc references CSS styles that control
the visual appearance. For a standard build of Qt's documentation (a
'local' build, if you will), an "offline" style is the default. This
style sheet is maintained in `qtbase.git`.

This style is not particularly well maintained, as it is not the style
in use for documentation published to `doc.qt.io`. As such, some
issues where found as a side-effect of other work. In particular, an
ill-placed background image in the `@media print` CSS instructions was
found in the print dialogue from locally generated content. While
validating the fix, it became clear that the `toc` CSS class was moved
into an obstructing position in the same media type.

Change the `@media print` definition such that:
- The `toc` class is hidden entirely.

Task-number: QTBUG-124162
Fixes: QTBUG-128205
Pick-to: 6.8
Change-Id: Ic427036a698d7fc0e07e9190289e5d2c196885d9
Reviewed-by: Andreas Eliasson <andreas.eliasson@qt.io>

doc/global/template/style/offline.css

@@ -642,14 +642,7 @@ Content table
 
 @media print {
     .toc {
-        float: right;
+        display: none;
-        clear: right;
-        padding-bottom: 10px;
-        padding-top: 50px;
-        width: 100%;
-        background-image: url(../images/bgrContent.png);
-        background-position: top;
-        background-repeat: no-repeat;
     }
 }