qa/tasks/keycloak: drop unused local variable

qa/tasks/keycloak: drop unused local variable

Regression introduced by ee710390d277784ddac3d70c9e11e427f46f363d
("rgw/tests: add os-specific java 1.7 install commands to keycloak
task").

```
283/322 Test #301: run-tox-qa ................................***Failed 92.31 sec
...
flake8: install_deps /ceph/qa> python -I -m pip install flake8
  flake8: commands[0] /ceph/qa> flake8 --select=F,E9 --exclude=venv,.tox
  ./tasks/keycloak.py:51:5: F841 local variable 'os_version' is assigned to but never used
```
Remove the unused os_version assignment to fix flake8 F841 in run-tox-qa.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>