Bug 1487091 - remove an unused element from a graphics IPDL union; r=aosmond

Differential Revision: https://phabricator.services.mozilla.com/D4543

--HG--
extra : moz-landing-system : lando

gfx/layers/ipc/LayersSurfaces.ipdlh

@@ -151,7 +151,6 @@ union SurfaceDescriptor {
   SurfaceDescriptorMacIOSurface;
   SurfaceDescriptorSharedGLTexture;
   SurfaceDescriptorGPUVideo;
-  SurfaceDescriptorShared;
   null_t;
 };