wallet_db: rm dependence on PaymentRequest class in convert_version_25

Change convert_version_25 to delete invoices instead of converting them.
convert_version_25 was released ~2 years ago. Wallet files not opened since will have old bip70 invoices deleted upon upgrading.

In general it is ~unsafe for convert_version_* to depend on other modules of the code.
(using e.g. sha256 is fine as its API will never change,
but using e.g. PaymentRequest is dangerous as its API might change over time)

electrum/paymentrequest.py

@@ -121,7 +121,7 @@ async def get_payment_request(url: str) -> 'PaymentRequest':
 
 class PaymentRequest:
 
-    def __init__(self, data, *, error=None):
+    def __init__(self, data: bytes, *, error=None):
         self.raw = data
         self.error = error  # FIXME overloaded and also used when 'verify' succeeds
         self.parse(data)
@@ -131,7 +131,7 @@ class PaymentRequest:
     def __str__(self):
         return str(self.raw)
 
-    def parse(self, r):
+    def parse(self, r: bytes):
         self.outputs = []  # type: List[PartialTxOutput]
         if self.error:
             return