You can uninstall or removes an installed amavisd-new-postfix package itself from Ubuntu 12.04 LTS (Precise Pangolin) through the terminal,
$ sudo apt-get remove amavisd-new-postfix
If you would like to remove amavisd-new-postfix and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove amavisd-new-postfix
If you use with purge options to amavisd-new-postfix package all the configuration and dependent packages will be removed.
$ sudo apt-get purge amavisd-new-postfix
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove amavisd-new-postfix