tester: drop obsolete qemu-kvm dependency

On sid (and thus soon Bullseye), qemu-system-x86 package has:

  Breaks: qemu-kvm
  Replaces: qemu-kvm

… so requiring to have both qemu-system-x86 and qemu-kvm installed
practically ensures we cannot run the test suite on sid.

qemu-kvm was only useful to provide the /usr/bin/kvm wrapper anyway,
which our test suite does not use, as it instead has:

  <emulator>/usr/bin/qemu-system-x86_64</emulator>

This is the Puppet equivalent of the corresponding change in tails/tails:
tails!91 (efe8ade2)