TranslationMergeJob migrates only POTMsgSets and no translations

TranslationMergeJob relies on TranslationMerger.mergePackagingTemplates to merge all the translations when a Packaging link is introduced. However, it only calls mergePOTMsgSets() without calling _removeDuplicateTranslations() or calling mergeTranslationMessages(), thus resulting in merged POTMsgSets but very weird situation where we have almost identical translation messages in sharing templates but which are distinct in the database.

This basically means that we are not using the benefits of the sharing until stuff is re-translated (and sometimes, due to data not being merged, not even then).