PreviewMediaFragment: Remove margin from top bar

Not needed since previous commit fixing scroll bugs, creates unwanted space

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

app/src/main/res/layout/fragment_preview_media.xml

@@ -26,7 +26,6 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:gravity="center"
-    android:layout_marginTop="?attr/actionBarSize"
     tools:context=".ui.preview.PreviewMediaFragment">
 
     <ImageView