Remove unused video _posts from teach.github.com migration

__Before:__
![screen shot 2013-12-04 at 8 56 52 am](https://f.cloud.github.com/assets/352082/1674673/d5e9012e-5cfc-11e3-82d6-d5c04310b11f.png)

__After:__
![screen shot 2013-12-04 at 8 58 15 am](https://f.cloud.github.com/assets/352082/1674684/e94dcc36-5cfc-11e3-811e-45a5733e3d90.png)