Remove LIBS from openacc_test in top-level Makefile

[gdicker1]

Since these OpenACC compilation tests don't depend on NetCDF, GPTL, or other libraries, problems can be prevented by removing the linking commands that come from the LIBS variable. For example, the openacc_test rule will fail and the build will abort if a library the compiler expects to link against hasn't built yet.

[gdicker1]

This fixes an issue that occurs when builds are attempted with SMIOL and OPENACC=true (assuming a target with valid *_ACC flags in the top-level Makefile). Since the SMIOL library hasn't been built at the early point in the build that the openacc_test rule is attempted, all attempts of these OpenACC compilation tests will fail without some modification to the LIBS variable (or removal of LIBS as this PR does).

E.g. this can be tested on NCAR's Derecho machine with module commands similar to below (note the lack of a module that provides PIO):

 module load ncarenv/23.06
 module load cuda
 module load nvhpc
 module load cray-mpich
 module load cray-libsci
 module load netcdf-mpi
 module load parallel-netcdf
 module load ncarcompilers

and the build command:

make nvhpc CORE=test OPENACC=true