Some misc clean up in the css.

The `background-position` is wrong here. Those two settings are here
almost from the beginning and we've rewrite the card template few time
since. By forcing the `top` position, we prevent the image to be centered.

The `#cards td label` is very old also and was used to make the label
in the card `clickable`. Now that the whole card is a link, this is
no more necessary (and already unused).
7 jobs for package_icon in 16 minutes and 26 seconds (queued for 11 minutes and 13 seconds)
latest
Status Job ID Name Coverage
  Test
passed #23940
Code Coverage

00:02:09

94.39%
passed #23941
Code Quality

00:00:44

passed #23942
Data Migration

00:07:02

passed #23943
Missing Migrations

00:00:31

passed #23939
Tests: C locale

00:02:30

passed #23937
Tests: Jessie/Python-3.4

00:01:55

passed #23938
Tests: Stretch/Python-3.5

00:02:04