Cleanup simulation reset in launch files

[ygoumaz]

Follow-up to conclusions in #655.

• webots_ros2_mavic and webots_ros2_tesla can be simplified to benefit from the respawn parameter.
• webots_ros2_epuck, webots_ros2_tiago and webots_ros2_turtlebot are cleaned-up to remove the reset functionality, as there is no clear solution to restart nav2 and rviz nodes. The manual solution with multiple launch files is documented in Limitations when resetting simulations #655 but won't be implemented in the package. However, it will also be documented in a new tutorial in the official ROS documentation on how to handle the reset button depending on the nodes of the package.
• webots_ros2_universal_robot is also dadapted for consistency.

[ygoumaz]

Test failures are not related to this PR, but to a change in Iron and Rolling testing.

[omichel]

Thank you.