BIP32_HD_Wallet: Fix address derivation

Unfortunately there was root_name and root_derivation confusion
in the past for classes derived from BIP_32_HD_Wallet.
Address derivation used root_name and so would begin with 'x/'
whereas it should have begun with root_derivation, and so started
with 'm/'.

This fixes that old wart and removes some fudges from the trezor
code that used to work around it.

plugins/trezor/plugin.py

@@ -100,7 +100,6 @@ class TrezorCompatibleWallet(BIP44_Wallet):
 
         # When creating a wallet we need to ask the device for the
         # master public key
-        derivation = derivation.replace(self.root_name, self.prefix() + "/")
         xpub = self.get_public_key(derivation)
         return xpub, None