Move back dirvote_authority_cert_dup to dirvote.c

Originally, it was made public outside of the dirauth module but it is no
longer needed. In doing so, we put it back in dirvote.c and reverted its name
to the original one:

dirvote_authority_cert_dup() --> authority_cert_dup()

Signed-off-by: David Goulet <dgoulet@torproject.org>