Remove calls to get thumbnail post.

[jenkoian]

When using this plugin with
https://github.com/humanmade/network-media-library it doesn't work, it
seems this is because it attempts to retrieve the post of the thumbnail
(needlessly afaict) which won't work because the image exists on a
different site in the network.

Removing these calls seems to fix the issue and allows for a better
mutli site experience. I'm not sure of any adverse affects of this, but
let me know if you can see one.

[jenkoian]

Alternatively we could define hooks at the start and end of these functions so integrators can switch/restore blog.

[jenkoian]

Here's an example of the hooks approach that I've just done for a similar issue on fieldmanager alleyinteractive/wordpress-fieldmanager#738 let me know if you'd prefer that route.