Committer's guide: Fix about (re)moving ports

Summary:

• Move consideration about security vulnerabilities from instructions for removing ports to instructions for moving ports.
• Reword the above consideration to take into account the splitting of vuln.xml into one file per year.

Reviewed by: portmgr (rene), Pau Amma <pauamma@gundo.com>
Approved by: docs (carlavilla), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D35112