pkgbase: Remove runtime dependency on bootloader

It isn't needed and will cause problem when upgrading to a newer release.

Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33878

(cherry picked from commit 5efa7281a790809e4109340f0e4adcfd0a68db49)