Remove signal_freebsd (already in pkg/signal)

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
Integration test for link and unlink containers

Docker-DCO-1.1-Signed-off-by: Fabio Falci <fabiofalci@gmail.com> (github: fabiofalci)
Merge pull request #5741 from creack/remove_unused_file-2

Remove signal_freebsd (already in pkg/signal)
Merge pull request #5729 from fabiofalci/link_unlink_test

Integration test for link and unlink containers