iRedMail ships script to uninstall iRedMail quickly now, and below are steps.
If it doesn't work for you, please post a new topic in iRedMail General Technical Support forum.

WARNING

It won't prompt you before stopping service, removing binary packages. So, please make sure you want to execute it.

Download shell script, and move it to iRedMail-x.y.z/tools/ directory.

# wget http://iredmail.googlecode.com/hg/extra/clear_iredmail.sh
# mv clear_iredmail.sh /path/to/iRedMail-x.y.z/tools/

Execute it

# cd /path/to/iRedMail-x.y.z/tools/
# bash clear_iredmail.sh

It need your confirm to remove binary packages and source tarballs (e.g. roundcube, phpmyadmin, phpldapadmi, etc), please read the prompt carefully and make decision.