make: remove mpi as SUBDIRS from test/Makefile.am

Having mpi as a SUBDIR in test/Makefile.am asks make to descend into
test/mpi when making targets suchas distclean. This fails since test/mpi
is no longer configured during the main mpich configure and
test/mpi/Makefile may not exist.

test/Makefile.am

@@ -6,7 +6,7 @@
 ## FIXME: commented out temporarily, really just want "mpi" to be dealt with at
 ## distclean time for now
 ##SUBDIRS = mpi util basic commands .
-SUBDIRS = mpi commands .
+SUBDIRS = commands .
 
 # Test both the MPI routines and the MPICH command scripts
 testing: mpi/Makefile