patch 8.1.1870: using :pedit from a help file sets help filetype

Problem:    Using :pedit from a help file sets the preview window to help
            filetype. (Wang Shidong)
Solution:   Do not set "keep_help_flag". (closes #3536)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index cea3936..941ad84 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -8612,16 +8612,16 @@
 {
     win_T	*curwin_save = curwin;
 
+    // Open the preview window or popup and make it the current window.
     g_do_tagpreview = p_pvh;
     prepare_tagpreview(TRUE);
 
-    keep_help_flag = bt_help(curwin_save->w_buffer);
+    // Edit the file.
     do_exedit(eap, NULL);
-    keep_help_flag = FALSE;
 
     if (curwin != curwin_save && win_valid(curwin_save))
     {
-	/* Return cursor to where we were */
+	// Return cursor to where we were
 	validate_cursor();
 	redraw_later(VALID);
 	win_enter(curwin_save, TRUE);