client: remove unnecessary chdir() calls in GPU detection

GPU detection code goes back and forth between program and data
directories but this is unnecessary because run_program() already takes
care of running the program in correct directory.

Fixes two "ignoring return value" warnings from GCC/GLIBC.

client/gpu_detect.cpp

@@ -661,8 +661,6 @@ int COPROCS::launch_child_process_to_detect_gpus() {
          NULL
     }; 
 
-    chdir(client_dir);
-
     retval = run_program(
         client_dir,
         client_path,
@@ -672,8 +670,6 @@ int COPROCS::launch_child_process_to_detect_gpus() {
         prog
     );
 
-    chdir(data_dir);
-
     if (retval) {
         if (log_flags.coproc_debug) {
             msg_printf(0, MSG_INFO,