remove dead code and silence compiler warnings

src/USER-OMP/fix_omp.cpp

@@ -80,7 +80,9 @@ FixOMP::FixOMP(LAMMPS *lmp, int narg, char **arg)
   if (nthreads < 1)
     error->all(FLERR,"Illegal number of OpenMP threads requested");
 
+#if defined(_OPENMP)
   int reset_thr = 0;
+#endif
   if (nthreads != comm->nthreads) {
 #if defined(_OPENMP)
     reset_thr = 1;