Howdy Amal,
I would love to help you with this.
The easiest way to trim that list would be to delete the test venue you created. If you go to WP-Admin > Events > Venues, find the Breuillet venue, and delete it, it should disappear from the front end.
If yoou wish to truly filter all venues with no event dynamically, you definitely could. But it will require a goodly measure of programming. If I were you i would extend the class Tribe__Events__Filterbar__Filters__City. Then you could add in your filtering code there, overriding any parent code you need to to facilitate the shorter list. Instantiate this new class of yours, and it will automatically add itself as a Filter Bar option in WP-Admin > Events > Settings > Filters. You can then change your settings on that page to show this new City filter instead of the old one.
Does that all make sense? Will that work for you? Please let me know.
Cheers!
– Brook