Is it possible to drop data from OLAP database (and to keep definitions)?
Yes, it is possible.

The easiest way would be to go with Management Studio to the proper SSAS server, right click on the database -> Script Database as -> Create to -> New query window.

That will generate a .XMLA file that creates the complete definition of your SSAS database. That's without the prepared data inside. When you need your database and its data, you just run the XMLA file, it will create the OLAP db, and then Process that database. This will add the data.

But, in theory at least, you should have a Visual Studio project where that SSAS database was created, so if you have it and it's the latest version, than you don't need the OLAP database at all, you just redeploy and process the data.