qa/suites/rados/perf/ceph.yaml: remove rgw

This is no longer required because we removed cosbench workloads in
fd350fd. This is also required to prevent
failures like the following or any other changes that break the rgw task:

```
2021-08-06T20:13:25.812 INFO:teuthology.orchestra.run.smithi060.stderr:curl: (7) Failed to connect to smithi060.front.sepia.ceph.com port 80: Connection refused
2021-08-06T20:15:33.813 ERROR:teuthology.contextutil:Saw exception from nested tasks
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_04c2febe7099917d97a71271f17abb5710030132/teuthology/contextutil.py", line 31, in nested
    vars.append(enter())
  File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/teuthworker/src/github.com_ceph_ceph-c_3c0f8c8164075af7aac4d1f2805d3f4580709461/qa/tasks/rgw.py", line 191, in start_rgw
    wait_for_radosgw(url, remote)
  File "/home/teuthworker/src/github.com_ceph_ceph-c_3c0f8c8164075af7aac4d1f2805d3f4580709461/qa/tasks/util/rgw.py", line 94, in wait_for_radosgw
    assert exit_status == 0
AssertionError
```

Signed-off-by: Neha Ojha <[email protected]>

qa/suites/rados/perf/ceph.yaml

@@ -15,5 +15,4 @@ tasks:
       - \(OSD_
       - \(OBJECT_
       - overall HEALTH
-- rgw: [client.0]
 - ssh_keys: