[client] remove all casts around malloc

The cast is unnecessary in C and should be removed to avoid clutter.

client/displayservers/X11/clipboard.c

@@ -153,7 +153,7 @@ static void x11CBReplyFn(void * opaque, LG_ClipboardData type,
 
 static void x11CBSelectionRequest(const XSelectionRequestEvent e)
 {
-  XEvent * s = (XEvent *)malloc(sizeof(*s));
+  XEvent * s = malloc(sizeof(*s));
   s->xselection.type      = SelectionNotify;
   s->xselection.requestor = e.requestor;
   s->xselection.selection = e.selection;