Fix memory leak in MET reconstruction algorithm

This fixes a memory leak in METUtilities. Previously there was a copy operation being used on a MissingETAssociationMap but the memory allocated for this was never freed. It turns out copying this at all is no longer necessary (it was originally to handle some const-ness issues which don't exist anymore), so I've simply removed it and directly used the original MissingETAssociationMap instead.

Tagging @wlampl (who reported this issue) and @williams, @khoo.