Remove uwsgi requirement

Even though we use uWSGI as our WSGI server for barbican,
we don't have any code dependencies.  It is possible to run
barbican usin gnuicorn or any other WSGI server, so we won't
require uWSGI in the pip-requires file.

Change-Id: Ia0f53cfea18a9c6d8087247c5c58114523437d50