requirements: Drop os-testr

os-testr has been decrepated [1], it's not necessary in a world with stestr.

[1]https://opendev.org/openstack/os-testr/src/branch/master/README.rst

Change-Id: Id2382f2c559ea7f4d4a629d137f07f0ce8841abc

Author: 1049965823

lower-constraints.txt

@@ -41,7 +41,6 @@ netifaces==0.10.4
 openstacksdk==0.53.0
 os-client-config==2.1.0
 os-service-types==1.7.0
-os-testr==1.0.0
 osc-lib==2.3.0
 oslo.concurrency==3.26.0
 oslo.config==5.2.0