That is interesting that it thinks it is still finding it. Our DRM keeps the entire file path, so if you moved the only copy you have to the trash, it shouldn't have found anything at the spot and offered to do a new install.
If you have trashed all existing copies of X-Plane, you can delete the DRM files found in: users/(your username)/Library/Preferences/. The files are: x-plane_install_10.txt, x-plane_drm.prf, x-plane_xdd.prf, x-plane installer.prf. Deleting these files will cause the installer to act like brand new no matter what you have on your computer.