login_cap.h: Remove LOGIN_DEFPRI

[olce]

Summary

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)
Sponsored by: The FreeBSD Foundation

[emaste]

Approved, pending all of the parent revs