thrift: fix open-source build
thrift: fix open-source build

Summary: The two `Callback.mustache` files would get `make install`ed into the same directory, which would make the install fail. fried says that since OSS Py3 is not ready yet, we might as well delete it for now.

Reviewed By: yfeldblum

Differential Revision: D5073403

fbshipit-source-id: aba4a8bcb93dff06abb8bb1163b68472e8861084

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,facebook/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift