You can uninstall or removes an installed default-mta package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove default-mta
If you would like to remove default-mta and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove default-mta
If you use with purge options to default-mta package all the configuration and dependent packages will be removed.
$ sudo apt-get purge default-mta
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 default-mta
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