Remove unneeded package sources from nodepool node

We need additional package sources on nodepool nodes to install things
like puppet, pypy, and python3. But once these packages are installed we
don't need them anymore. Any test that does an apt-get update or yum
upgrade will hit these package repos despite not needing to. Remove the
additional sources after we are done with them to be nicer to them.

Fixes-Bug: 1253774
Change-Id: I3c5bd7e544076f26b224553bdea6134d39fc8bba
Merge "Remove unneeded package sources from nodepool node"