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/fold.c b/src/fold.c
index 73c7116..3f8de1c 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -2637,7 +2637,7 @@
 			// to stop just above startlnum.
 			fp->fd_len = startlnum - fp->fd_top;
 			foldMarkAdjustRecurse(&fp->fd_nested,
-				(linenr_T)fp->fd_len, (linenr_T)MAXLNUM,
+				fp->fd_len, (linenr_T)MAXLNUM,
 						       (linenr_T)MAXLNUM, 0L);
 			fold_changed = TRUE;
 		    }