extensionDownloader: Stop handling 'blacklist' operation

Blacklist support was added all the way back in commit 1e286e43, but
the code had been defunctional until recently. While uninstalling an
extension that has been blacklisted makes sense off-hand, unfortunately
we don't know if an extension was *actually* blacklisted:

The website returns that operation for any extensions for which it
doesn't find any versions that match the shell version. That is, the
most likely reason is that the user updated to a new GNOME release
which the extension doesn't support yet.

It doesn't look like the website is going to change that behavior any
time soon[0], so drop the 'blacklist' handling for the time being.

[0] Infrastructure/extensions-web#95

GNOME/gnome-shell!1132
8 jobs for !1132 with no-blacklist in 7 minutes and 18 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Review
passed #642061
check_commit_log

00:01:31

passed #642063
eslint

00:02:10

passed #642062
js_check

00:00:33

passed #642065
no_template_check

00:00:39

passed #642064
potfile_check

00:00:43

 
  Build
passed #642066
build

00:03:14

 
  Test
passed #642067
test

00:01:53

passed #642068
test-pot

00:01:52