patch 8.2.4241: some type casts are redundant

Problem:    Some type casts are redundant.
Solution:   Remove the type casts. (closes #9643)
diff --git a/src/clipboard.c b/src/clipboard.c
index e2ebd11..9eace3d 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -1488,7 +1488,7 @@
 	// create NUL terminated string which XmbTextListToTextProperty wants
 	mch_memmove(string_nt, string, (size_t)*length);
 	string_nt[*length] = NUL;
-	conv_result = XmbTextListToTextProperty(X_DISPLAY, (char **)&string_nt,
+	conv_result = XmbTextListToTextProperty(X_DISPLAY, &string_nt,
 					   1, XCompoundTextStyle, &text_prop);
 	if (conv_result != Success)
 	{