bsdinstall/zfsboot: Remove unused global variable

In an AMA session on the FreeBSD Discord, it was revealed that no
logic remains calling GPART_BOOTCODE_PARTONLY. It was removed in
aa2a0e0fc311 for FreeBSD 12. git grep shows no other references.
No functional change intended.

MFC after: 2 weeks
Fixes: aa2a0e0fc311 (Enable new UEFI+GELI support)
Reported by: Antranig Vartanian <antranigv@freebsd.am>
Reviewed by: emaste, mhorne
Approved by: emaste, mhorne (src)
Approved by: mhorne, carlavilla (mentors)
Differential Revision: https://reviews.freebsd.org/D48086