refactor(fixtures): remove unused pkg_resources (#142)

Remove the unused `pkg_resources` import that was not being used
anywhere in the code base. Its removal helps to eliminate deprecation
warnings.

Closes reanahub/reana-commons#502

Author: tiborsimko

pytest_reana/fixtures.py

@@ -16,7 +16,6 @@
 import shutil
 from uuid import uuid4
 
-import pkg_resources
 import pytest
 from kombu import Connection, Exchange, Queue
 from kubernetes import client