hw plugins: (cleanup) Plugin objects should not have a Client field

Clients are per-connected-device, plugins are ~singletons.
These were mostly remnants of old code.

electrum/plugins/digitalbitbox/digitalbitbox.py

@@ -661,7 +661,6 @@ class DigitalBitboxPlugin(HW_PluginBase):
 
     libraries_available = DIGIBOX
     keystore_class = DigitalBitbox_KeyStore
-    client = None
     DEVICE_IDS = [
                    (0x03eb, 0x2402) # Digital Bitbox
                  ]

electrum/plugins/ledger/ledger.py

@@ -581,7 +581,6 @@ class Ledger_KeyStore(Hardware_KeyStore):
 class LedgerPlugin(HW_PluginBase):
     keystore_class = Ledger_KeyStore
     minimum_library = (0, 1, 32)
-    client = None
     DEVICE_IDS = [
                    (0x2581, 0x1807), # HW.1 legacy btchip
                    (0x2581, 0x2b7c), # HW.1 transitional production