Use REST DELETE API for deleting reviewers

Pushes more of the error message generation to the server side.
Because the DELETE request returns no content, we need to make a
second RPC to refresh the approval table after the delete succeeds.
Just removing the row from the table is not sufficient since the list
of needed labels may have changed.

Change-Id: I1d31c9d6c29d268994ce6df3f59310231324ec0e
2 files changed