No longer check for new git-review releases

* git-review(latest_is_newer,main,print_exit_message)
(update_latest_version): The git-review tool is now being packaged
actively by most operating systems/distributions popular amongst the
current user base, and has been stable without serious usability
bugs for some time. As a result, the expectation is that most users
are now installing from places other than PyPI and so the somewhat
hackish functions which check for and warn users about the existence
of new releases have outlived their usefulness. Removing them to
pave the way for continued ease of maintainability improvements, and
also to be one less annoying thing packagers and users have to
disable downstream.

Closes-Bug: 1212829
Change-Id: I9d2ddea794f2b53d4137f0916de4afa029120f59
Migrate to pbr.

For ease of future source code maintenance by OpenStack developers,
git-review is now adopting the same Python Build Reasonableness as
used by current OpenStack projects.

Closes-Bug: 1179007
Change-Id: I20427b51e84b90dcc1d870a547cdcc5240b4ff0e