librbd: remove unwanted std::move

librbd: remove unwanted std::move

Clang argues:
```
/home/jenkins/workspace/ceph-master-compile/src/librbd/migration/HttpClient.cc:463:16: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
    response = std::move(m_parser->release());
               ^
/home/jenkins/workspace/ceph-master-compile/src/librbd/migration/HttpClient.cc:463:16: note: remove std::move call here
    response = std::move(m_parser->release());
               ^~~~~~~~~~                   ~
1 warning generated.

```

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>