Document the removal of old ports

• databases/rubygem-dbd-sqlite: removed by dinoex in r368734.
• devel/rubygem-multipart-post: removed by antoine in r458121.
• www/rubygem-jquery-ui-rails-rails5: removed by antoine in r446377.

It helps portscout.FreeBSD.org remove the stale entries [1].

Reference: https://portscout.FreeBSD.org/ruby@freebsd.org.html [1]