Remove unnecessary line in drawRRT.cpp

I think the below code work for nothing.
~~~
fprintf(gnuplot, "");
~~~

This commit removes compiler warning:
~~~
/home/js/DRCDev/dart_master/apps/drawRRT/drawRRT.cpp: In function 'void draw(const dart::planning::RRT*, const dart::planning::RRT*)':
/home/js/DRCDev/dart_master/apps/drawRRT/drawRRT.cpp:97:22: warning: zero-length gnu_printf format string [-Wformat-zero-length]
   fprintf(gnuplot, "");
                      ^
~~~