remove omp.h includes in DiapOTB

Maybe I'm wrong, but I don't think OpenMP is still used in DiapOTB, so omp.h an be removed from the remote module.

Right now an OTB build with DiapOTB will fail if these headers cannot be found, even if OTB is builts without OpenMP support

I compiled OTB and DiapOTB succesfully with and without OpenMP with this commit, and ran the tests succesfully.