Removes reference to codecov token since no longer needed

[marshallmcdonnell]

Since the repository is now public, we no longer need the CODECOV_TOKEN, so removing it in the GitHub Actions yaml

NOTE: We also need to delete this from the GitHub Secrets as well. @stuchalk, I believe you will have to do this based on permissions.

To test:

• This is the Actions pipeline that shows coverage upload passed: https://github.com/ChalkLab/SciDataLib/runs/2336335554?check_suite_focus=true
• Also, this commit shows up as the most recent in the CodeCov dashboard and the specific commit report is here

[stuchalk]

The secret has been removed... don't tell anyone :)