Don't directly pip install tempest pip-requires

This code:

    # Tempest doesn't satisfy its dependencies on its own, so
    # install them here instead.
    pip_install -r $TEMPEST_DIR/tools/pip-requires

causes us to do:

    Found existing installation: oslo.config 1.1.0
     Uninstalling oslo.config:

so rather than 1.1.0 installed, we end up with 1.1.0b1 and everything
goes kabloomy.

It looks like this comment came from a time (commit 9a352daf) before
tempest included proper install_requires (tempest commit 5abb253c3).

It should be safe to delete this and allow setup_develop() do all the
work just like the other projects.

Change-Id: I869f727900bb08c72e5e8f4e84858a9c77049715