The mbuf_tell, mbuf_rewind and pgp_get_cipher_name functions were
introduced in commit e94dd6ab91, but were never used, so remove.
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/FCF3F14E-17D5-41F2-AC58-0A97B341193A@yesql.se