pythonPackages.{reviewboard,djblets}: uninit

Djblets is unmaintained: has not been updated since 2015, but had many releases.
Dependency django_pipeline_1_3 is broken and should anyway be removed from pythonPackages because we want to have a consistent package set.
Because the reviewboard package also hasn't been updated since 2015 and depends on djblets, it is removed as well.