The following guide will teach you how to completely remove the plugin from Redmine. You have to follow these steps:
Downgrade your database
Please navigate to the Redmine directory and execute the following command:bundle exec rake redmine:plugins NAME=redmine_contacts VERSION=0 RAILS_ENV=production
Delete the plugin's folder
To do so, please execute the following command as still being in the redmine/ directory:rm -fr plugins/redmine_contacts
Restart
Please restart your web server.