Revert "Use gtk clipboard when available to avoid https://bugreports.qt.io/browse/QTBUG-56595"

Fixed in Qt by https://codereview.qt-project.org/c/qt/qtbase/+/306771

This reverts commit 3a91003eea.

Telegram/SourceFiles/platform/linux/linux_gtk_integration_dummy.cpp

@@ -33,10 +33,6 @@ bool GtkIntegration::showOpenWithDialog(const QString &filepath) const {
 	return false;
 }
 
-QImage GtkIntegration::getImageFromClipboard() const {
-	return {};
-}
-
 QString GtkIntegration::AllowedBackends() {
 	return {};
 }