Consistency in resource format saver/loader de-registration

Some used 'is_valid()' checks, others not. Validity is already checked in 'unref()',
and 'remove_resource_format_*()' has an ERR_FAIL condition on 'is_null()' already
(which shouldn't happen since we're only unregistering things that we previously
registered.

Also add missing GDCLASS statement in ResourceFormatLoaderVideoStreamGDNative,
missed in #20552 which was last amended before #19501 was merged.

modules/gdnative/videodecoder/video_stream_gdnative.h

@@ -197,6 +197,7 @@ public:
 };
 
 class ResourceFormatLoaderVideoStreamGDNative : public ResourceFormatLoader {
+	GDCLASS(ResourceFormatLoaderVideoStreamGDNative, ResourceFormatLoader)
 public:
 	virtual RES load(const String &p_path, const String &p_original_path = "", Error *r_error = NULL);
 	virtual void get_recognized_extensions(List<String> *p_extensions) const;