Remove testscenarios denpendency

Since the commit 482e3ce6ab had
just removed the NegativeAutoTest Framework, which was the only
user of package testscenarios, testscenarios is no longer used by
tempest. This patch suggests to remove testscenarios from the
package denpendency defined file requirments.txt.

Change-Id: Id2d4ab7c54223044551e4684e9ae1eee130f01c0

requirements.txt

@@ -15,7 +15,6 @@ oslo.serialization>=1.10.0 # Apache-2.0
 oslo.utils>=3.16.0 # Apache-2.0
 six>=1.9.0 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD
-testscenarios>=0.4 # Apache-2.0/BSD
 PyYAML>=3.10.0 # MIT
 python-subunit>=0.0.18 # Apache-2.0/BSD
 stevedore>=1.17.1 # Apache-2.0