tests: Remove tox's specific environment setup

0528e04 removed the need for it so that both pytest/tox would work.
Prior to 0528e04, only tox worked.

resources/loader.yml

-storage:
-  cls: memory
-  args: {}

tox.ini

@@ -5,8 +5,6 @@ envlist=flake8,py3
 deps =
   .[testing]
   pytest-cov
-setenv =
-   SWH_CONFIG_FILENAME = ./resources/loader.yml
 commands =
   pytest --cov=swh --cov-branch {posargs}