Values from Locations ... From Field "Assets::Location
The value list is getting the values from data that is already in the field "Assets::Location" in other records. There are 2 ways you can get rid of the values you don't want in the list.
1. Change the type of value list. It could be a custom value list - you type in the values that you want to have appear in the dropdown. You could have a separate table with a record for each location and the value list gets its values from that field. You could create a "current location" flag and create a table occurrence that only shows current locations.
2. Change the data in your records so that you only have assets in the locations that you want. eg. if you currently have 20 locations showing in your dropdown list, then across all your records those 20 locations are in use. Update records so that across all records you only have 10 locations and you'll find you only have 10 locations in the dropdown list.
In a field you can press Command + i keys (mac) or Control + i keys (windows) to show the index of the field - it will show what values are currently in use in that field across all your records. Do a find on any location that is not current and then you can put in the correct locations for those records.