lncfg: remove deprecated PrivateTowerURIs from Wtclient cfg

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.

sample-lnd.conf

@@ -992,11 +992,6 @@ litecoin.node=ltcd
 ; specified in sat/byte, the default is 10 sat/byte.
 ; wtclient.sweep-fee-rate=10
 
-; (Deprecated) Specifies the URIs of private watchtowers to use in backing up
-; revoked states. URIs must be of the form <pubkey>@<addr>. Only 1 URI is
-; supported at this time, if none are provided the tower will not be enabled.
-; wtclient.private-tower-uris=
-
 ; The range over which to choose a random number of blocks to wait after the
 ; last channel of a session is closed before sending the DeleteSession message
 ; to the tower server. The default is currently 288. Note that setting this to