Stop installing deb.torproject.org-keyring

The purpose of this package is to keep deb.torproject.org's APT key up-to-date,
which does not work in Tails as we discourage users from updating packages.

On the build/dev side, this package only adds complexity and confusion: we have
to install the APT key ourselves anyway (otherwise live-build would not be able
to install packages we install from deb.torproject.org, before this keyring
package is installed). So this only adds a second place where we need to
maintain the same information, with no benefit. For example, I updated the key
in bfcd3b23, but the deb.torproject.org-keyring
package from the APT snapshot we're using replaces it, in the built images, with
an older, expired one.
5 jobs for !269 with bugfix/18042-update-deb.torproject.org-key in 3 minutes and 59 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Test
passed #1934
test-iuk

00:03:26

passed #1935
test-perl5lib

00:02:46

passed #1936
test-persistence-setup

00:03:58

passed #1937
test-python-doctest

00:00:21

passed #1938
test-whisperback

00:01:01