Remove get_service_instance from ros_loader

[kenji-miyake]

Public Changes

None

Description

Since services can't be instantiated in ROS2, I removed the function.
I've noticed this in #644.

Related: #644

How to test
ROS1

$ echo $ROS_DISTRO
noetic

$ python -c "from std_srvs.srv import Trigger; print(Trigger())"
<std_srvs.srv._Trigger.Trigger object at 0x7f42024894c0>

ROS2

$ echo $ROS_DISTRO
galactic

$ python -c "from std_srvs.srv import Trigger; print(Trigger())"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/ros/galactic/lib/python3.8/site-packages/std_srvs/srv/_trigger.py", line 278, in __init__
    raise NotImplementedError('Service classes can not be instantiated')
NotImplementedError: Service classes can not be instantiated

[amacneil]

thanks!