Remove references to non-existing files AddExclusionDialog.* and IStatsEventHandler.h

Description ----------- There are files that have been deleted already in previous commits that are still referenced in the make files. These are AddExclusionDialog.cpp, AddExclusionDialog.h, AddExclusionDialog.ui and IStatsEventHandler.h. These missing files are what keeps flathubbot from building the version 5.4, see the following references: - https://github.com/flathub/nz.mega.MEGAsync/pull/90 - https://buildbot.flathub.org/#/builders/25/builds/17024