Remove unused GetType() from OverrideStream, CVectorWriter, SpanReader

GetType() is never called, so it is completely unused and can be
removed.

src/wallet/test/wallet_tests.cpp

@@ -752,14 +752,14 @@ bool malformed_descriptor(std::ios_base::failure e)
 BOOST_FIXTURE_TEST_CASE(wallet_descriptor_test, BasicTestingSetup)
 {
     std::vector<unsigned char> malformed_record;
-    CVectorWriter vw(0, 0, malformed_record, 0);
+    CVectorWriter vw{0, malformed_record, 0};
     vw << std::string("notadescriptor");
     vw << uint64_t{0};
     vw << int32_t{0};
     vw << int32_t{0};
     vw << int32_t{1};
 
-    SpanReader vr{0, 0, malformed_record};
+    SpanReader vr{0, malformed_record};
     WalletDescriptor w_desc;
     BOOST_CHECK_EXCEPTION(vr >> w_desc, std::ios_base::failure, malformed_descriptor);
 }