macOS: Remove DontConfirmOverwrite workaround

Does not work any more on recent macOS versions.

As a bonus, Qt now handles file names which contain
“___qt_very_unlikely_prefix_” correctly.

Task-number: QTBUG-39791
Change-Id: I944a68efa18edc72939d953ab32ecb53d8f8e1c4
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit fded886ff4b9869c1a9e016e3fae05cc409c9ccd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qcocoafiledialoghelper.mm

@@ -331,14 +331,7 @@ static QString strippedText(QString s)
 - (NSString *)panel:(id)sender userEnteredFilename:(NSString *)filename confirmed:(BOOL)okFlag
 {
     Q_UNUSED(sender);
-    if (!okFlag)
-        return filename;
-    if (!mOptions->testOption(QFileDialogOptions::DontConfirmOverwrite))
-        return filename;
-
-    // User has clicked save, and no overwrite confirmation should occur.
-    // To get the latter, we need to change the name we return (hence the prefix):
-    return [@"___qt_very_unlikely_prefix_" stringByAppendingString:filename];
+    return filename;
 }
 
 - (void)setNameFilters:(const QStringList &)filters hideDetails:(BOOL)hideDetails
@@ -396,7 +389,7 @@ static QString strippedText(QString s)
         if (fileInfo.suffix().isEmpty() && !defaultSuffix.isEmpty()) {
                 filename.append('.').append(defaultSuffix);
         }
-        result << QUrl::fromLocalFile(filename.remove(QLatin1String("___qt_very_unlikely_prefix_")));
+        result << QUrl::fromLocalFile(filename);
         return result;
     }
 }