This commit removes the `PrivateTowerURIs` member from the `WtClient`
config struct. This field has been deprecated since v0.8.0-beta and
currently, LND would fail to start if the field was specified.
In this commit, a `DefaultWtClientCfg` function is added which
populates some default values for the `WtClient` struct. This is then
used to populate the wtclient defaults in the main LND config struct.
With the introduction of the WatchtowerClient RPC subserver, the lnd
configuration flag to specify private watchtowers for the client is no
longer needed and can lead to confusion upon users. Therefore, we remove
the flag completely, and only rely on the watchtower client being active
through a new --wtclient.active flag.