Stop appending default host if none is provided.

Appending the default hostname broke backward
compatibility. This change reverses that change.
Now, usernames with @ in them will need to
specify their host deliberately, but that is
a small edge case compared to every single
user call being modified without the client user's
knowing.

Fixes Bug #1171559

Change-Id: I2f2e9adc252e2b1cd03b1bb0c4a53d48d5c11e53
Merge "Stop appending default host if none is provided."