Extra WPCom REST requests on self hosted sites

[maxme]

I was checking Stetho network requests while testing this #11522 and I found that we're trying to fetch diffs from wpcom REST API even if a self hosted site (non jetpack) is selected.

It's not a big deal, but we're kind of leaking self hosted usage via this extra call.

Expected behavior

No network call to wpcom REST API when a non-jetpack self hosted site is selected.

Steps to reproduce the behavior

• Select a self hosted site.
• Go to the post list.
• Tap on a published post.

Tested on [device], Android [version], WPAndroid [version]