Remove no-longer-used integration marker

Author: mhucka

dev_tools/conf/pytest.ini

@@ -15,10 +15,6 @@
 [pytest]
 markers =
     slow: marks tests as slow (deselect with '-m "not slow"')
-    integration: marks tests of integrations with other systems
 
 filterwarnings =
     ignore:Skipped assert_qasm_is_consistent_with_unitary because qiskit.*:UserWarning
-
-# Add this line to always exclude tests marked as 'integration' by default
-addopts = -m "not integration"