login_cap.h: Remove LOGIN_DEFUMASK

This public constant has not been used in-tree since 1997 (this was
noticed while working on previous commit "setusercontext(): umask: Set
it only once (in the common case)").

Since it was an implementation detail and GitHub's code search and
Google show no use of this symbol today, simply remove it.

PR: 276570 (exp-run)
Reviewed by: emaste, kib (earlier version, then part of D40344)
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43608