control: Drop Build-Depends on qemu-system-common

Up until recently, the qemu-{bridge,pr}-helper binaries included
in that package needed to be available at build time in order to
discover their non-standard location; as of libvirt 9.4.0,
however, that lookup is performed at runtime rather than build
time so we no longer need this Build-Depends.

debian/control

@@ -57,7 +57,6 @@ Build-Depends:
  po-debconf,
  python3-docutils,
  python3:native,
- qemu-system-common:native,
  qemu-utils,
  systemtap-sdt-dev [linux-any],
  xsltproc,