Remove unused variable.

Reported by @adrianreber, this patch removes an unused variable in the
PSM MTL, silencing a compiler warning.
(cherry picked from commit open-mpi/ompi@e7bcad0c130c9324b45504894ec0ebeb590fdbaf)
Merge pull request #99 from afriedle-intel/topic/afriedle-psm-warning-fix-v1.8

Remove unused variable.