[rootci] Use system nlohmann/json.hpp on opensuse16

opensuse16 includes `nlohmann_json-devel` package which
should be enough for ROOT functionality

Author: linev

.github/workflows/root-ci-config/buildconfig/opensuse16-march_native.txt

@@ -5,7 +5,6 @@ builtin_tbb=ON
 builtin_vdt=ON
 builtin_xrootd=ON
 builtin_unuran=ON
-builtin_nlohmannjson=ON
 test_distrdf_dask=OFF
 test_distrdf_pyspark=OFF
 tmva-pymva=ON