Is there a way to delete unused fields? I created many fields for content type.Now I don't need these fields anymore. 
I deleted the fields from the content type but the fiels data remains in the database.

How can I delete the field and all its related data?
Sometimes running cron does not entirely delete a field, especially if the field was ever used in a field collection. First, make sure the field isn't used anywhere else. Check all content types, field collections, custom user fields, etc. if the field is really not used but running cron does not delete the field table from the database, you may have to delete it manually. You can do this manually with a SQL query or via phpmyadmin.