Use JSON_TestDataDirectory directly

Author: Gianfranco Costamagna

cmake/download_test_data.cmake

@@ -1,12 +1,6 @@
 set(JSON_TEST_DATA_URL     https://github.com/nlohmann/json_test_data)
 set(JSON_TEST_DATA_VERSION 3.1.0)
 
-set(JSON_TEST_DATA_DIR /usr/share/json_test_data)
-
-if(EXISTS ${JSON_TEST_DATA_DIR})
-  set(JSON_TestDataDirectory ${JSON_TEST_DATA_DIR})
-endif()
-
 # if variable is set, use test data from given directory rather than downloading them
 if(JSON_TestDataDirectory)
     message(STATUS "Using test data in ${JSON_TestDataDirectory}.")