Committers-guide port section: remove addport

Summary:
The script addport is not working anymore, it's based on SVN and
it's quite hard to port it to git.
Before its removal, we want to remove its references in the
documentation.

Related bugzilla entry:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261378

Reviewers: tcberner, se

Differential Revision: https://reviews.freebsd.org/D35071