Remove LINK_MAX.

After recent changes to change filesystems to use filesystem-specific
limits, LINK_MAX is no longer used in the base system. Applications
should in theory be able to cope with a lack of LINK_MAX by using
pathconf().

PR: 224628 (exp-run)
Approved by: imp, kib
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D13658