scheduler: Remove useless type assertions
scheduler: Remove useless type assertions

The return type of join.Join is already compatible with the return type
of syncWorker, so there is no reason to iterate and convert the values
manually.

NetSys/quilt,LuiseV/quilt,LuiseV/quilt,kelda/kelda,kelda/kelda,kklin/quilt,NetSys/quilt,kklin/kelda,LuiseV/quilt,quilt/quilt,kklin/kelda,ejj/quilt-dev,kklin/quilt,LuiseV/quilt,quilt/quilt,kklin/kelda,NetSys/quilt,quilt/quilt,ejj/quilt-dev,kelda/kelda,NetSys/quilt,kklin/quilt,ejj/quilt-dev,quilt/quilt,ejj/quilt-dev,kklin/quilt,NetSys/quilt,kelda/kelda,kklin/kelda,LuiseV/quilt