retire the bunq integration

The webhook is deregistered at bunq and the callback endpoint, its parser,
default-deny classifier, dedup ledger and signature check are removed; the
code is in git history if a bank feed ever comes back. /financials keeps
reading the hand-maintained aggregates file, and sales + shop donations
stay live from the order ledger.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/harness/Catcrafts.E2eHarness.cppm

@@ -108,7 +108,7 @@ struct ServerOptions {
     // attach the signed invoice, shell out — runs with zero network. Each
     // accepted message lands as its own mail-<n>.eml.
     bool mailer = false;
-    // Extra environment for the server (e.g. BUNQ_CALLBACK_SECRET).
+    // Extra environment for the server.
     std::vector<std::pair<std::string, std::string>> env;
 };
 
@@ -129,7 +129,7 @@ public:
         // suites assert.
         for (const char* v : { "MOLLIE_API_KEY", "EURC_CHAINS", "EURC_POOL",
                                "SENDCLOUD_PUBLIC_KEY", "SENDCLOUD_SECRET_KEY",
-                               "SENDCLOUD_METHOD", "BUNQ_CALLBACK_SECRET",
+                               "SENDCLOUD_METHOD",
                                "INVOICE_GPG_KEY", "MAIL_COMMAND", "MAIL_FROM" }) {
             ::unsetenv(v);
         }