MFC r342597:

Remove legacy rc.d infrastructure references from rc(8)

Legacy rc.d scripts (.sh extension) have not been supported since
r193118. Remove the outdated references to the legacy format, as they
are no longer valid.

Bug: 193936
Approved by: emaste (earlier version, mentor), jtl (mentor)
Differential Revision: https://reviews.freebsd.org/D18804