Remove workaround for pytest

[ruffsl]

Now that cited issues are closed. See:

• Pytest issue on Ubuntu 18.04 ros2/ros2_documentation#271
• pytest not found causing build to fail ros2/ros2#722

[mikaelarguedas]

These lines act as integration tests as well. They ensure that pytest is installed and found properly. I think there is value in keeping them to catch future breakages early

[ruffsl]

These lines act as integration tests as well. They ensure that pytest is installed and found properly.

Not sure how the current directive halts the image build if the incorrect pytest version is found, vs. if it's not found at all.

I think there is value in keeping them to catch future breakages early

Ok. Lets let it be then.

[mikaelarguedas]

We had an issue that some versions were incompatible preventing python from finding pytest at all.
This ensures that pytest can be found / invoked meaning there is a "working" version. We do not enforce version number as long as pytest can be used