Remove interface filtering from the plugin factory

This doesn't work that way, and CIDs really are class IDs (good thing
that everything's documented, oh right).

src/plugin/vst3-plugin.cpp

@@ -14,14 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-#include <public.sdk/source/main/pluginfactory.h>
-
 #include "bridges/vst3.h"
 
 #include <public.sdk/source/main/linuxmain.cpp>
 
-using Steinberg::gPluginFactory;
-
 // Because VST3 plugins consist of completely independent components that have
 // to be initialized and connected by the host, hosting a VST3 plugin through
 // yabridge works very differently from hosting VST2 plugin. Even with