You can uninstall or removes an installed r-mathlib package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove r-mathlib
If you would like to remove r-mathlib and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove r-mathlib
If you use with purge options to r-mathlib package all the configuration and dependent packages will be removed.
$ sudo apt-get purge r-mathlib
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 r-mathlib