json_db: exempt keystore from StoredDict conversion

The keystore logic would need to be significantly changed to nicely
interoperate with StoredDict/json_db logic.
(just see KeyStore.__init__() and KeyStore.dump())
For now we exempt the keystore from the recursive StoredDict conversion, as
it is a smaller change that is also easier to review for correctness.

fixes #6066
fixes #6401

also reverts 2d3c2eeea9 (which was an even hackier workaround for #6066)

electrum/plugins/bitbox02/bitbox02.py

@@ -521,7 +521,7 @@ class BitBox02_KeyStore(Hardware_KeyStore):
     device = "BitBox02"
     plugin: "BitBox02Plugin"
 
-    def __init__(self, d: StoredDict):
+    def __init__(self, d: dict):
         super().__init__(d)
         self.force_watching_only = False
         self.ux_busy = False

electrum/plugins/ledger/ledger.py

@@ -255,7 +255,6 @@ class Ledger_KeyStore(Hardware_KeyStore):
         self.force_watching_only = False
         self.signing = False
         self.cfg = d.get('cfg', {'mode': 0})
-        self.cfg = dict(self.cfg)  # convert to dict from StoredDict (see #6066)
 
     def dump(self):
         obj = Hardware_KeyStore.dump(self)