Remove incorrect operator!= for GncItem

It's better to use the default std::pair implementation

- spotted by clang-tidy bugprone-suspicious-string-compare
- will always return false, because type is the same

Also,
- remove redundant "using GncItem ..."

libgnucash/engine/gnc-option-impl.cpp

@@ -143,8 +143,6 @@ GncOptionGncOwnerValue::serialize() const noexcept
     return retval;
 }
 
-using GncItem = std::pair<QofIdTypeConst, GncGUID>;
-
 static GncItem
 make_gnc_item(const QofInstance* inst)
 {
@@ -176,14 +174,6 @@ qof_instance_from_gnc_item(const GncItem& item)
     return static_cast<QofInstance*>(qof_collection_lookup_entity(coll, &guid));
 }
 
-static bool
-operator!=(const GncItem& left, const GncItem& right)
-{
-    auto [ltype, lguid]{left};
-    auto [rtype, rguid]{right};
-    return strcmp(rtype, ltype) && !guid_equal(&rguid, &lguid);
-}
-
 GncOptionQofInstanceValue::GncOptionQofInstanceValue(
     const char* section, const char* name,
     const char* key, const char* doc_string,
@@ -233,6 +223,12 @@ GncOptionQofInstanceValue::reset_default_value()
     m_value = m_default_value;
 }
 
+static bool
+operator==(const GncGUID& l, const GncGUID& r)
+{
+    return guid_equal(&l, &r);
+}
+
 bool
 GncOptionQofInstanceValue::is_changed() const noexcept
 {
@@ -447,12 +443,6 @@ GncOptionAccountListValue::get_default_value() const
     return retval;
 }
 
-static bool
-operator==(const GncGUID& l, const GncGUID& r)
-{
-    return guid_equal(&l, &r);
-}
-
 bool
 GncOptionAccountListValue::is_changed() const noexcept
 {