make-flight: further reduce unwanted combinations

make-flight: further reduce unwanted combinations

Drop armhf build and test jobs from tests of branches:
  - qemu-upstream-unstable
  - qemu-upstream-4.2-testing
  - qemu-upstream-4.3-testing
  - linux-3.10
  - linux-3.4

In principal these are harmless but in practice they are stretching our
available armhf build resources very thin. In addition spurious armhf failures
now won't affect these branches.

Drop non-armhf arch builds and tests from linux-arm-xen.