Revert "Add pytest-cov install even when coverage is not requested"

This reverts commit e7f538d693f41db9198bd4dfaa3a8695f17e67a9.
pytest-cov is no longer mandatory when the e3.pytest plugin is used
but only when `--cov=..` is passed.

ref it/e3-core#29

See merge request it/cookiecutter-e3!79 of the template

Author: cookiecutter-e3

tox.ini

@@ -9,7 +9,7 @@ passenv = CI,GITHUB_*,CODECOV_*
 
 deps =
      xdist: pytest-xdist[psutil]
-     pytest-cov
+     cov: pytest-cov
      codecov: codecov
 extras = test