CTO Purge
This add-on adds a new button to Blender's Transformation Orientation panel which empowers the user to purge all of his/her custom transformation orientations with one button click.
Installation
First get the addon by clicking the green "Code" button (top right of files list above) and select the "Download ZIP" option, then use Blender's own "Install..." (button) option after navigating to Blender's Main Menu --> Edit --> Preferences... --> Add-ons.