Remove bin/authorized_keys

We committed to removing this script in the 13.x release cycle, and it was announced then (see gitlab-com/www-gitlab-com!50054 (merged) ), but we never actually got around to it.

Its functionality is completely replicated, in a safer manner, by the bin/gitlab-shell-authorized-keys script.

Prior context in #209 (closed) . We should have created this MR before closing that issue.