don't assign tx_api when not needed

tests/common.py

@@ -11,7 +11,6 @@ class TrezorTest(unittest.TestCase):
         self.client = TrezorDebugClient(self.transport)
         self.client.set_debuglink(self.debug_transport)
         # self.client.set_buttonwait(3)
-        self.client.set_tx_api(TXAPIBitcoin())
 
         #                     1      2     3    4      5      6      7     8      9    10    11    12
         self.mnemonic12 = 'alcohol woman abuse must during monitor noble actual mixed trade anger aisle'

trezorctl

@@ -442,7 +442,6 @@ def main():
     else:
         client = TrezorClient(transport)
 
-    client.set_tx_api(TXAPIBitcoin())
     cmds = Commands(client)
     
     res = args.func(cmds, args)

trezorlib/client.py

@@ -538,7 +538,10 @@ class ProtocolMixin(object):
                 continue
 
             tx = msg.transactions.add()
-            tx.CopyFrom(self.tx_api.get_tx(binascii.hexlify(inp.prev_hash)))
+            if self.tx_api:
+                tx.CopyFrom(self.tx_api.get_tx(binascii.hexlify(inp.prev_hash)))
+            else:
+                raise Exception('TX_API not defined')
             known_hashes.append(inp.prev_hash)
 
         return msg
@@ -560,7 +563,10 @@ class ProtocolMixin(object):
             if inp.prev_hash in known_hashes:
                 continue
 
-            txes[inp.prev_hash] = self.tx_api.get_tx(binascii.hexlify(inp.prev_hash))
+            if self.tx_api:
+                txes[inp.prev_hash] = self.tx_api.get_tx(binascii.hexlify(inp.prev_hash))
+            else:
+                raise Exception('TX_API not defined')
             known_hashes.append(inp.prev_hash)
 
         return txes