Remove the rpm entry in requirements.txt

[mulkieran]

requirements.txt is used to supply additional requirements for Travis tasks. Ultimately we would be better off getting rid of requirements.txt entirely. The basic reason for this is that the requirements for the different Travis tasks are not shared by all the tasks. For example, the yapf requirement only applies to the formatting task. Moreover, we are discouraged from using tox at all by the Travis docs, as when running on Python, Travis already makes a virtual environement, see related: stratis-storage/project#36. In the meantime, though, we should get rid of an entry that is not used by the Travis tasks or by anything else that we can figure out.