Use a getter for a proxy object's instance ID

We also need this to get some random other object's instance ID, so
might as well use it everywhere.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -39,8 +39,6 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
     tresult PLUGIN_API queryInterface(const Steinberg::TUID _iid,
                                       void** obj) override;
 
-    inline size_t instance_id() { return arguments.instance_id; }
-
     // From `IAudioProcessor`
     tresult PLUGIN_API
     setBusArrangements(Steinberg::Vst::SpeakerArrangement* inputs,