Remove the unnecessary sync() and the restrictions it imposes on 3D Physics.

modules/bullet/bullet_physics_server.cpp

@@ -1553,9 +1553,6 @@ void BulletPhysicsServer3D::step(float p_deltaTime) {
 	}
 }
 
-void BulletPhysicsServer3D::sync() {
-}
-
 void BulletPhysicsServer3D::flush_queries() {
 	if (!active) {
 		return;

modules/bullet/bullet_physics_server.h

@@ -397,7 +397,6 @@ public:
 
 	virtual void init() override;
 	virtual void step(float p_deltaTime) override;
-	virtual void sync() override;
 	virtual void flush_queries() override;
 	virtual void finish() override;