Remove Wyre buy-provider integration (shut down 2023)

Delete the WyreBuyProvider + WyreOrderProviderAdapter source files,
strip imports and call sites from buy/order view models and the
buy webview, force isBitcoinBuyEnabled = false in di.dart, and drop
the three wyreSecretKey/wyreApiKey/wyreAccountId entries from
tool/utils/secret_key.dart.

BuyProviderDescription.wyre (raw: 0) is kept as a no-op legacy entry
so any old Hive-persisted order with that raw value still deserializes
without throwing. Don't wire it back to UI/services.

tool/utils/secret_key.dart

@@ -14,9 +14,6 @@ class SecretKey {
     SecretKey('backupKeychainSalt', () => hex.encode(encrypt.Key.fromSecureRandom(12).bytes)),
     SecretKey('changeNowCakeWalletApiKey', () => ''),
     SecretKey('changeNowMoneroApiKey', () => ''),
-    SecretKey('wyreSecretKey', () => ''),
-    SecretKey('wyreApiKey', () => ''),
-    SecretKey('wyreAccountId', () => ''),
     SecretKey('moonPayApiKey', () => ''),
     SecretKey('moonPaySecretKey', () => ''),
     SecretKey('sideShiftAffiliateId', () => ''),