Remove cone shape from particle emitter (gazebosim#446)

Signed-off-by: Ian Chen <[email protected]>

Author: iche033

proto/gz/msgs/particle_emitter.proto

@@ -55,8 +55,6 @@ message ParticleEmitter
     CYLINDER  = 2;
     /// \brief Ellipsoid emitter.
     ELLIPSOID = 3;
-    /// \brief Cone emitter.
-    CONE = 4;
   }
   /// \brief The emitter type.
   EmitterType type              = 4;