Fix generation of trigger configurations

There is a consistency check in PropertyConfig::initProperties, which attempts to verify that toString / fromString are each others inverse. With Gaudi v27r1 (2016-patches), the check does not seem to work (fromString seems to have no effect on the property). (edit: the check does seem to be working, but the update handler does not seem to be passed to the clone ) . With Gaudi v28r0, this check is actually working and fromString triggers a call to the property's update handler.

The latter is a problem for LoKi filters, because it triggers the functor decoding, which can't succeed at this stage of execution (e.g. see this nighly):

HltGenConfig                              ERROR failure in update handler of 'Preambulo': HltPV3D:: Unable to decode the functor!

My solution is to remove the update handler from the cloned property before doing the check. This will potentially miss bad behaviour in the update handler, like modifying the property value.