Revert element link creation for trigger matching

While migrating the DF trigger matching tool to use ReadHandles I inadvertently switched to using the handle's hashed key (which hashes using the CLID for xAOD::IParticleContainer) from letting the link read it from the event store (which then hashes with the correct CLID). This apparently makes these links unreadable in AnalysisBase. The fix is to give the link the std::string key which it then uses to figure out the correct hash behind the scenes.

This should close ATR-26071