rdb.set_trace stops in contextlib

The ignore_errno context manager there didn't suppress anything as
Pdb.set_trace does not raise an ECONNRESET in any case. After the
removal, the method was the same as Pdb.set_trace, so this just
removes Rdb's override.

The test also asumed that pdb.set_trace might raise ECONNRESET even though this is not possible.