login_cap.h: Remove LOGIN_DEFPRI

This is an implementation detail which is likely to become irrelevant in
the future, as we move to not resetting the priority if the
corresponding capability is not present in the configuration file
('/etc/login.conf').

GitHub's code search and Google show no use of this public constant, and
it doesn't exist in OpenBSD and NetBSD.

So, remove this definition and its sole use in-tree.

PR: 276570 (exp-run)
Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43609

(cherry picked from commit c7e645269f1c7e9872a7ccf6b72f045c55a66793)

Approved by: markj (mentor)