patch 8.2.2598: Vim9: :open does not need to be supported

Problem:    Vim9: :open does not need to be supported.
Solution:   Do not support :open in Vim9 script.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 40e70c3..c7b57ba 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -6609,6 +6609,10 @@
     regmatch_T	regmatch;
     char_u	*p;
 
+#ifdef FEAT_EVAL
+    if (not_in_vim9(eap) == FAIL)
+	return;
+#endif
     curwin->w_cursor.lnum = eap->line2;
     beginline(BL_SOL | BL_FIX);
     if (*eap->arg == '/')