Remove some debug output for Git command execution.

This debug output was introduced in a prior change. It printed the
command being executed (which already exists in logging), and the result
(which does not). It's been removed.
Make post-review show a deprecation warning.

This adds a deprecation warning to the 0.5.x branch, to warn people that
post-review is going away in the next major release.

Testing done:
Installed from the release-0.5.x branch and ran post-review. Saw the
warning.

Reviewed at https://reviews.reviewboard.org/r/5201/
Merge branch 'release-0.5.x'
Merge branch 'release-0.5.x'
Update .reviewboardrc for the release-0.5.x branch.
Merge branch 'release-0.5.x'
Remove post-review.

This change removes post-review from rbtools.

Testing done:
* Built and installed a package.
* Built the docs.

Reviewed at https://reviews.reviewboard.org/r/5202/