This code comes from oslo-incubator and is now frozen,
and the client module specifically is not used in mistralclient.
It's confusing to have in the library since mistralclient has
it's own HTTPClient under mistralclient.api.httpclient. So let's
just remove this effectively dead code.
Change-Id: If20d05b6675448578be1a5162208007470c7dddb