remove instructions on flattening vendor trees

[emaste]

I had a look at https://wiki.freebsd.org/ContribSoftware and I believe all contrib software that needed flattening is done, possibly excluding netcat (which I'll check, and flatten if necessary).

[emaste]

Taking another approach, the following vendor/*/dist/contrib directories exist:

• bind
• csup
• diff
• global
• hostapd
• isc-dhcp
• libgmp
• libiberty
• libpam
• libreadline
• ngatm
• opie
• pam_modules
• perl5
• pnpinfo
• smbfs
• tcl
• tcp_wrappers
• telnet
• traceroute

These also have a contrib/ directory, but as part of upstream:

• ficl
• 
• ldns
• libarchive
• sendmail
• unbound

[emaste]

sigh apparently can't type groff