Fix performance regression when doing property lookups for non-existant properties

Commit 84627464

 introduced a performance
regression of falling back to reading the QMetaObject, when the lookup for a
property in QML failed, after we've tried in the property cache. This is very
very expensive to do and was only due to QQmlPropertyMap not correctly
invalidating the property cache.

Instead remove the property cache from the property map's QObject and on the
lookup side rely on the property cache being correct in the result (positive or
negative).

Change-Id: I8a013483203f2007d48b71feafa10b3ea02c53fd
Reviewed-by: Lars Knoll <lars.knoll@digia.com>