hier(7): remove text describing /usr/src layout

It poses a maintenance burden, since much of the information is
duplicated in the src tree's README.md file. Readers who are interested
enough in learning about the structure of the src tree can download it,
or browse the README online. Have hier(7) just point them there instead.

PR: 261349
Discussed with: freebsd-arch@, freebsd-doc@ lists
Reviewed by: imp, emaste
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37135