lib: Drop igt_child_done

Added in

commit 054eb1ab
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Thu Mar 30 14:32:29 2017 +0100

    benchmarks/gem_wsim: Command submission workload simulator

but since then the only user was lost.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>