Remove #ifdef GMX_THREAD_MPI for basic thread safety.

Simplifies the code, and at least most of this should have zero impact
on performance.  Basic thread support from thread-MPI is already
required for compilation anyways even with GMX_THREAD_MPI off.
This removes about 70% of the #ifdefs, making it clearer what is
different between MPI and thread-MPI implementation in Gromacs.

Also set MPI_IN_PLACE_EXISTS with GMX_THREAD_MPI instead of checking for
GMX_THREAD_MPI separately each time.  Adjust one #ifdef in network.c to
replace an GMX_LIB_MPI made unnecessary by this with GMX_MPI.

Related to #948 and #1320.

Change-Id: I03a54b0bffde090100afe5ed2e91c184af36c189