Don't show preview picture in article view (duplicate preview)

Same as wallabag/wallabag#1875

Repro:
0. fetch article https://dilbert.com/strip/2019-01-21
0. view article in android app

expected: see one image
actual: see 2 duplicated images

Unfortunately, I don't have the android skills to find this in the code and figure out how to remove it. Hoping someone can pick it up, and make android app consistent with web app.

# Environment details
- **wallabag app version**: 2.2.0 (210)
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): Gplay
- **Android OS version**:
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…):
- **Android hardware**:
- **wallabag server version**: 2.3.7
- **Do you have Two-Factor-Authentication enabled?**: no

# Your experience with wallabag Android app
app is nice! I especially like the ability to the favorite, unread, and archive tabs for specific tags, which isn't available in web UI.