Possible to remove unused modes from GUI drop downs?

I would love to streamline some of the drop-downs, especially mode, to only list certain modes (especially removing Manual mode). Is there a way to do that?

Well, it is possible but has drawbacks.
Mission Planner uses Ardupilot parameter definition files to determine flight modes for a given platform. These files are in the ProgramData/Mission Planner hidden directory. MP uses the FLTMODE1 parameter definition to get a list of flightmodes for a platform. You can edit the XML file and remove the modes you want to hide. BUT : MP updates the files in every seven days, this will overwrite your modification (if you are online) plus when you remove a mode that will be non existent to MP, so it will display it as UNKNOWN on the HUD.