coroutine: Drop co_sleep_ns

block_job_sleep_ns is the only user. Since we are moving towards
AioContext aware code, it's better to use the explicit version and drop
the old one.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>