Clean .gitlab-ci.yaml from leftovers after `tox` running

Michał Kudelarequested to merge
kudmich/clean-ci.yaml-from-Tox-leftovers into develop
  1. Remove references to the no longer used tox package in the gitlab-ci.yaml file,
  2. Remove PYTHONPATH gitlab-ci.yaml file.

Related to:

@bwrona After merge issue can be closed #408 (closed)

Edited by Michał Kudela

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