It is possible to remove this drop down from within expenses if you follow the below steps:
Click Admin Menu or Configuration, depending on how your system is configured.
Click into Settings then click Field Settings.
Search for FormData_Mileage_m.ProjectId.
Change display to Disabled then click Save.
📌Note: Now that you have disabled this field setting, when you go to add a mileage expense this field will no longer display.