Remove the job queue drain rpc call

This call was introduced but never used. In two years.
Since it's just creating/removing a file it can also be in simpler ways,
without a special rpc call, if/when we need it again. In the meantime,
let's give it to history.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>