module: zfs: arc: hdr_full_crypt_dest: drop unevaulated-only variable

This explodes as -Wunused-variable on GCC 8.5.0, despite it being used,
just not in an evaluated context

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13195