src_c/IMB_chk_diff.c: Remove unused rank variable

[shijin-aws]

This fixes the compilation error on unused variable.

make[1]: Leaving directory '/home/ubuntu/PortaFiducia/build/workloads/imb/mpi-benchmarks/src_cpp'
../src_c/IMB_chk_diff.c: In function ‘IMB_chk_diff’:
../src_c/IMB_chk_diff.c:462:34: error: unused variable ‘rank’ [-Werror=unused-variable]
  462 |     int         j, j1, j2, ierr, rank, allpos;
      |                                  ^~~~
mpicc -g -O0 -Wall -Werror -DCHECK -Ihelpers -I../src_c -DNBC -I. -DNBC -c -o NBC/IMB_reduce.o ../src_c/IMB_reduce.

[shijin-aws]

@JuliaRS can u take a look?