Drop indirect dependency

The testtools library is not directly used. It's an indirect dependency
required by oslotest.

Change-Id: I07a09ae940fcb2872d06783b2eb80f4def66c6a4
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

HACKING.rst

@@ -52,7 +52,7 @@ bug that had no unit test, a new passing unit test should be added. If a
 submitted bug fix does have a unit test, be sure to add a new one that fails
 without the patch and passes with the patch.
 
-All unittest classes must ultimately inherit from testtools.TestCase.
+All unittest classes must ultimately inherit from oslotest.base.BaseTestCase.
 
 All setUp and tearDown methods must upcall using the super() method.
 tearDown methods should be avoided and addCleanup calls should be preferred.

test-requirements.txt

@@ -2,6 +2,5 @@ fixtures>=3.0.0 # Apache-2.0/BSD
 oslo.messaging[kafka]>=8.0.0 # Apache-2.0
 oslotest>=3.8.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
-testtools>=2.2.0 # MIT
 stestr>=2.0.0 # Apache-2.0
 testresources>=2.0.1 # Apache-2.0