fix: remove Opus from default codec set, add non-CGo stub

- Remove OpusSDPName from init() default codec set to enforce opt-in design
- Add media_codecs_opus_nocgo.go with no-op SetOpusEnabled for non-CGo builds

Fixes nocgo build failures and ensures Opus is disabled by default.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thetillhoff

pkg/sip/media_codecs_opus_nocgo.go

@@ -0,0 +1,20 @@
+// Copyright 2024 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !cgo
+
+package sip
+
+// SetOpusEnabled is a no-op in non-CGo builds; Opus requires libopus.
+func SetOpusEnabled(_ bool) {}