I have answered my own questions:
To rebuild/recreate the MPD database, I :
- deleted var/lib/mpd/mpd.db
rebooted pi (I'm guessing that restarting MPD would have worked)
went to Sources and clicked on "Rebuild MPD Library"
I discovered that the reason the deleted content is still in the database is because there is a hidden folder on the USB drive that Windows creates/uses. (See
https://support.microsoft.com/en-us/kb/171694). I use a windows PC to update/edit the music on this device.
I mounted the drive on a Mac and was able to see/delete the folder. I reattached the drive on the Pi and rebult the library. The deleted music is now gone. If I could only figure out how to mount just the music folder instead of the whole drive, I could easily avoid this problem.