[MAINT] Simplify maintenance of `FindDO.cmake`.

We don't use the template file `FindDO.cmake.in` anymore:
- Using such template is hardly justified as we just replace the version value of DO-CV in the placeholder `@DO_VERSION@` in `FindDO.cmake.in`!
- Disabling a temporarily broken library is a pain as we must remember to modify the template file `FindDO.cmake.in` instead of `FindDO.cmake`, which gets overwritten when we regenerate the project with CMake.

The second point is particularly annoying as it hinders the continuous integration process.