test/ librbd: clean up unused 'TEST_COOKIE'

test/ librbd: clean up unused 'TEST_COOKIE'

Clean them up because
```
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/test/librbd/exclusive_lock/test_mock_PreAcquireRequest.cc:47:26: error: unused variable 'TEST_COOKIE' [-Werror,-Wunused-const-variable]
static const std::string TEST_COOKIE("auto 123");
                         ^
1 error generated.
```

Ref: https://jenkins.ceph.com/job/ceph-pull-requests-arm64/59242/consoleFull#-5120798632a811ea2-3e7b-466b-84b4-d13df7e35809

Signed-off-by: Rongqi Sun <sunrongqi@huawei.com>