[Qt] remove unused PaymentRequestPlus::getPKIType function

src/qt/paymentrequestplus.h

@@ -29,7 +29,6 @@ public:
     bool SerializeToString(std::string* output) const;
 
     bool IsInitialized() const;
-    QString getPKIType() const;
     // Returns true if merchant's identity is authenticated, and
     // returns human-readable merchant identity in merchant
     bool getMerchant(X509_STORE* certStore, QString& merchant) const;