souporcell_env.yaml: Remove *sklearn* provided already by *scikit-learn*

Putting the packages from `souporcell_env.yaml` in `requirements.txt` and
installing them with Pip results in:

    $ pip3 install -r requirements.txt
    […]
    Collecting sklearn (from -r requirements.txt (line 63))
      Downloading sklearn-0.0.post12.tar.gz (2.6 kB)
      Installing build dependencies ... done
      Getting requirements to build wheel ... error
      error: subprocess-exited-with-error

      × Getting requirements to build wheel did not run successfully.
      │ exit code: 1
      ╰─> [15 lines of output]
          The 'sklearn' PyPI package is deprecated, use 'scikit-learn'
          rather than 'sklearn' for pip commands.

          Here is how to fix this error in the main use cases:
          - use 'pip install scikit-learn' rather than 'pip install sklearn'
          - replace 'sklearn' by 'scikit-learn' in your pip requirements files
            (requirements.txt, setup.py, setup.cfg, Pipfile, etc ...)
          - if the 'sklearn' package is used by one of your dependencies,
            it would be great if you take some time to track which package uses
            'sklearn' instead of 'scikit-learn' and report it to their issue tracker
          - as a last resort, set the environment variable
            SKLEARN_ALLOW_DEPRECATED_SKLEARN_PACKAGE_INSTALL=True to avoid this error

          More information is available at
          https://github.com/scikit-learn/sklearn-pypi-package
          [end of output]

      note: This error originates from a subprocess, and is likely not a problem with pip.
    error: subprocess-exited-with-error

As *scikit-learn* is already listed, remove *sklearn*.

souporcell_env.yaml

@@ -108,7 +108,6 @@ dependencies:
     - scikit-learn==0.21.3
     - scipy==1.3.0
     - six==1.12.0
-    - sklearn==0.0
     - tenacity==5.0.4
     - tensorboard==1.14.0
     - tensorflow==1.14.0