Remove test_network_quotas_scenario

This is not really a scenario test. Scenarios are supposed to
validate workflows.

Also, network quotas are already covered in tempest.api.network and
this scenario does not work well with parallelism enabled.

Change-Id: I8b363bc1a7ac40173ba76ca41c8a78a3032edb02
Merge "Remove test_network_quotas_scenario"