Fix Windows build by removing two unused declarations (#4708)

Remove the `verify` and `message` function declarations. The explicit
instantiation requests could not be completed because there were no
implementations for those two member functions. It is helpful that the
Microsoft (MSVC) compiler on Windows appears to be strict when it comes
to template instantiation.

This resolves the warning:

  XChainAttestations.h(450): warning C4661: 'bool
  ripple::XChainAttestationsBase<ripple::XChainClaimAttestation>::verify(void)
  const': no suitable definition provided for explicit template
  instantiation request

src/ripple/protocol/XChainAttestations.h

@@ -437,16 +437,6 @@ public:
     template <class T>
     void
     emplace_back(T&& att);
-
-    // verify that all the signatures attest to transaction data.
-    [[nodiscard]] bool
-    verify() const;
-
-protected:
-    // Return the message that was expected to be signed by the attesters given
-    // the data to be proved.
-    [[nodiscard]] std::vector<std::uint8_t>
-    message() const;
 };
 
 template <class TAttestation>