Remove a bug in mpi fmm for dealloc memory.

Valgrind says there is not memory lack (excepted because of openmp)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/scalfmm/scalfmm/trunk@78 2616d619-271b-44dc-8df4-d4a8f33a7222