qdoc: Clean up the code for managing read-only status for QML properties

QmlPropertyNode class has a duplicated API for resolving the read-only status of a property;
QmlPropertyNode::isReadOnly() and QmlPropertyNode::isWritable().

The logic is a bit convoluted and depends on call sites to call the correct one, depending whether the read-onlyness is expected to be resolved from an associated C++ property or not.