Remove unnecessary use of css_class.

Now that all packages have a thumbnail coming from the catalog,
we don't need anymore the `css_class`, and neither to store all
the icons ourselves in our code base.
7 jobs from package_icon_only in 7 minutes and 56 seconds (queued for 28 minutes and 40 seconds)
latest
Status Job ID Name Coverage
  Test
passed #23947
Code Coverage

00:01:48

94.49%
passed #23948
Code Quality

00:00:28

passed #23949
Data Migration

00:04:11

passed #23950
Missing Migrations

00:00:28

passed #23946
Tests: C locale

00:01:34

passed #23944
Tests: Jessie/Python-3.4

00:01:40

passed #23945
Tests: Stretch/Python-3.5

00:02:18