Remove --experimental transitional_allow_list

This flag was removed from fidlc in https://fxrev.dev/1000412.

Change-Id: Iaa7c1a03fef7988bc32b7f87236d189b35254535
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/1009608
Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>
diff --git a/backend/server.go b/backend/server.go
index 91311b7..1f37c54 100644
--- a/backend/server.go
+++ b/backend/server.go
@@ -399,7 +399,6 @@
 		for _, availableArg := range strings.Fields(r.Options.VersionSelection) {
 			args = append(args, "--available", availableArg)
 		}
-		args = append(args, "--experimental", "transitional_allow_list")
 		args = append(args, fileArgs...)
 		run, err := s.fidlc.run(ctx, args...)
 		if err != nil {