remove deprecated a-model (#1525)

Author: ngokevin

src/extras/primitives/index.js

@@ -8,12 +8,11 @@ require('./primitives/a-curvedimage');
 require('./primitives/a-cylinder');
 require('./primitives/a-image');
 require('./primitives/a-light');
-require('./primitives/a-model');
 require('./primitives/a-obj-model');
 require('./primitives/a-plane');
 require('./primitives/a-ring');
-require('./primitives/a-sound');
 require('./primitives/a-sky');
+require('./primitives/a-sound');
 require('./primitives/a-sphere');
 require('./primitives/a-torus');
 require('./primitives/a-torus-knot');