pyln: Remove deprecated txprepare variant

We promised to deprecate the old "destination satoshi feerate utxos"
variant some time on mid-2020 so let's do just that. `txprepare`
doesn't parse it either anymore.

Changelog-Changed: pyln: `txprepare` no longer supports the deprecated `destination satoshi feerate utxos` call format.

contrib/pyln-client/pyln/client/lightning.py

@@ -1172,19 +1172,7 @@ class LightningRpc(UnixDomainSocketRpc):
 
         return self.call("withdraw", payload)
 
-    def _deprecated_txprepare(self, destination, satoshi, feerate=None, minconf=None):
+    def txprepare(self, outputs, feerate=None, minconf=None, utxos=None):
-        warnings.warn("txprepare now takes output arg: expect removal"
-                      " in Mid-2020",
-                      DeprecationWarning)
-        payload = {
-            "destination": destination,
-            "satoshi": satoshi,
-            "feerate": feerate,
-            "minconf": minconf,
-        }
-        return self.call("txprepare", payload)
-
-    def txprepare(self, *args, **kwargs):
         """
         Prepare a Bitcoin transaction which sends to [outputs].
         The format of output is like [{address1: amount1},
@@ -1194,13 +1182,6 @@ class LightningRpc(UnixDomainSocketRpc):
         Outputs will be reserved until you call txdiscard or txsend, or
         lightningd restarts.
         """
-        if 'destination' in kwargs or 'satoshi' in kwargs:
-            return self._deprecated_txprepare(*args, **kwargs)
-
-        if len(args) and not isinstance(args[0], list):
-            return self._deprecated_txprepare(*args, **kwargs)
-
-        def _txprepare(outputs, feerate=None, minconf=None, utxos=None):
         payload = {
             "outputs": outputs,
             "feerate": feerate,
@@ -1209,8 +1190,6 @@ class LightningRpc(UnixDomainSocketRpc):
         }
         return self.call("txprepare", payload)
 
-        return _txprepare(*args, **kwargs)
-
     def txdiscard(self, txid):
         """
         Cancel a Bitcoin transaction returned from txprepare. The outputs