Drop X11 root properties for KDE full session / session version

From what I can tell these aren't used by anybody, and not something we
want to be using whilst we increase our wayland support.

Grepping through all of KDE, the only usage I found was kde 4's
KApplication and that code is exclusively checking for
"KDE_SESSION_VERSION==4" which will be false anyway.
The version in kdelibs4support checks for the env variable too.

startkde/startplasma-waylandsession.cpp

@@ -41,7 +41,6 @@ int main(int argc, char **argv)
     // Keep for KF5; remove in KF6 (KInit will be gone then)
     runSync(QStringLiteral("kdeinit5_shutdown"), {});
 
-    cleanupX11();
     out << "startplasma-waylandsession: Done.\n";
 
     return 0;

startkde/startplasma-x11.cpp

@@ -93,7 +93,6 @@ int main(int argc, char **argv)
     runSync(QStringLiteral("kdeinit5_shutdown"), {});
 
     cleanupPlasmaEnvironment(oldSystemdEnvironment);
-    cleanupX11();
 
     out << "startkde: Done.\n";
 

startkde/startplasma.cpp

@@ -336,28 +336,6 @@ void setupX11()
     //     so don't move this up.
 
     runSync(QStringLiteral("xsetroot"), {QStringLiteral("-cursor_name"), QStringLiteral("left_ptr")});
-    runSync(QStringLiteral("xprop"),
-            {QStringLiteral("-root"),
-             QStringLiteral("-f"),
-             QStringLiteral("KDE_FULL_SESSION"),
-             QStringLiteral("8t"),
-             QStringLiteral("-set"),
-             QStringLiteral("KDE_FULL_SESSION"),
-             QStringLiteral("true")});
-    runSync(QStringLiteral("xprop"),
-            {QStringLiteral("-root"),
-             QStringLiteral("-f"),
-             QStringLiteral("KDE_SESSION_VERSION"),
-             QStringLiteral("32c"),
-             QStringLiteral("-set"),
-             QStringLiteral("KDE_SESSION_VERSION"),
-             QStringLiteral("5")});
-}
-
-void cleanupX11()
-{
-    runSync(QStringLiteral("xprop"), {QStringLiteral("-root"), QStringLiteral("-remove"), QStringLiteral("KDE_FULL_SESSION")});
-    runSync(QStringLiteral("xprop"), {QStringLiteral("-root"), QStringLiteral("-remove"), QStringLiteral("KDE_SESSION_VERSION")});
 }
 
 void cleanupPlasmaEnvironment(const std::optional<QStringList> &oldSystemdEnvironment)

startkde/startplasma.h

@@ -26,7 +26,6 @@ void importSystemdEnvrionment();
 void runEnvironmentScripts();
 void setupPlasmaEnvironment();
 void cleanupPlasmaEnvironment(const std::optional<QStringList> &oldSystemdEnvironment);
-void cleanupX11();
 bool syncDBusEnvironment();
 void setupFontDpi();
 QProcess *setupKSplash();