patch 8.2.1882: Vim9: v:disallow_let is no longer needed

Problem:    Vim9: v:disallow_let is no longer needed.
Solution:   Remove v:disallow_let.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 934b4d6..7ae47b3 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -7319,12 +7319,8 @@
 		    break;
 
 	    case CMD_let:
-		    if (get_vim_var_nr(VV_DISALLOW_LET))
-		    {
-			emsg(_(e_cannot_use_let_in_vim9_script));
-			break;
-		    }
-		    // FALLTHROUGH
+		    emsg(_(e_cannot_use_let_in_vim9_script));
+		    break;
 	    case CMD_var:
 	    case CMD_final:
 	    case CMD_const: