I will be the bearer of bad news. Apparently there is not such feature. In their help page about the activities (where you can see a bunch of your "activities") nor in any other aggregator you can do this. Maybe you can mess with the API's as suggest this question in StackOverflow you will manage, otherwise, using the web application, keep clicking for hours.
For the API:
Access to this link: https://developers.facebook.com/tools/explorer/?method=GET&path=me%2Flikes%3Ffields%3Did, generate a token using the button at the top right part of the screen.
In the user permissions make sure
user_likes is selected:
Click "Get", now you will get a bunch of ID's about all your likes, take note of each number, then use the following URL to delete them:
https://graph.facebook.com/ID?method=delete issue several post request. You can select your preferred application to do it (using the access token) or the Graph API Explorer as I did.