Revert "build-rina: also test on latest Debian release"

The Debian packaging of our RINA kernel builds with GCC 4.9,
which is not available on Stretch:

  # apt install ./linux-build-deps_4.1.33+rina65.g2e1cdbcb4-1_amd64.deb
  ...
  The following packages have unmet dependencies:
  linux-build-deps : Depends: gcc-4.9 but it is not installable

software/build-rina/software.cfg

@@ -72,7 +72,7 @@ sparse-checkout = /playbook/roles/rina
 [vm-debian]
 # building a generic Debian kernel uses a lot of space
 size = 16Gi
-dists += debian-jessie
+dists = debian-jessie
 packages +=
 # generic (another SR that build packages automatically would use the same list)
   apt-utils build-essential devscripts equivs lsb-release