X11: Fix returned clipboard string freed too early

The clipboard string should not be freed on SelectionClear.  The user
may have received it from glfwGetClipboardString and it should remain
valid until the next call to a public clipboard string function.

src/x11_window.c

@@ -865,20 +865,6 @@ static Atom writeTargetToProperty(const XSelectionRequestEvent* request)
     return None;
 }
 
-static void handleSelectionClear(XEvent* event)
-{
-    if (event->xselectionclear.selection == _glfw.x11.PRIMARY)
-    {
-        _glfw_free(_glfw.x11.primarySelectionString);
-        _glfw.x11.primarySelectionString = NULL;
-    }
-    else
-    {
-        _glfw_free(_glfw.x11.clipboardString);
-        _glfw.x11.clipboardString = NULL;
-    }
-}
-
 static void handleSelectionRequest(XEvent* event)
 {
     const XSelectionRequestEvent* request = &event->xselectionrequest;
@@ -1171,12 +1157,7 @@ static void processEvent(XEvent *event)
         return;
     }
 
-    if (event->type == SelectionClear)
-    {
-        handleSelectionClear(event);
-        return;
-    }
-    else if (event->type == SelectionRequest)
+    if (event->type == SelectionRequest)
     {
         handleSelectionRequest(event);
         return;
@@ -1853,10 +1834,6 @@ void _glfwPushSelectionToManagerX11(void)
                     handleSelectionRequest(&event);
                     break;
 
-                case SelectionClear:
-                    handleSelectionClear(&event);
-                    break;
-
                 case SelectionNotify:
                 {
                     if (event.xselection.target == _glfw.x11.SAVE_TARGETS)