release: de-duplicate arm.subr fstab logic

These days, the entries are the identical for GPT and MBR.

Reviewed by: manu, karels, imp
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43847

(cherry picked from commit 94b86c12f14e69ace1bffe23c15c77a7def23216)