patch 8.2.3460: some type casts are not needed

Problem:    Some type casts are not needed.
Solution:   Remove unnecessary type casts. (closes #8934)
diff --git a/src/buffer.c b/src/buffer.c
index 507eb01..5616487 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -103,7 +103,7 @@
     retval = readfile(
 	    read_stdin ? NULL : curbuf->b_ffname,
 	    read_stdin ? NULL : curbuf->b_fname,
-	    (linenr_T)line_count, (linenr_T)0, (linenr_T)MAXLNUM, eap,
+	    line_count, (linenr_T)0, (linenr_T)MAXLNUM, eap,
 	    flags | READ_BUFFER);
     if (retval == OK)
     {