Additional dl flag is not needed when calling target_link_libraries() for lttng

I recently looked at the content of LTTNG_LIBRARIES here: https://gitlab.com/micro-ROS/ros_tracing/ros2_tracing/blob/7047550ee257fff8980592be838fa3f0ae5b9354/tracetools/CMakeLists.txt#L57

and it contains dl already: lttng-ust;dl

Therefore the additional dl can most likely be removed.