Remove disused /cdrom toplevel dir from hier(7).

While here, fix igor and mandoc -T lint low-hanging fruit in hier(7)

Reviewed by: danfe (D37473), gjb (mentor)
Approved by: gjb (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37472
Differential Revision: https://reviews.freebsd.org/D37473

(cherry picked from commit 720e0e0ff1d3d47bd8a5a7bb23353e4b5da71048)
(cherry picked from commit 88be8b8e54797db2fc0c3134890d1918aea2c087)