Remove old diff/merge sources (look in the Git history if you need them)

.../old/org/eventb/texttools/diff/EventBAttributesCheck.java

...tools/old/org/eventb/texttools/diff/EventBDiffEngine.java

.../old/org/eventb/texttools/diff/EventBReferencesCheck.java

...ols/old/org/eventb/texttools/merge/EventBMatchEngine.java

...b.texttools/old/org/eventb/texttools/merge/MergeUtil.java

...ttools/old/org/eventb/texttools/merge/MergerProvider.java

.../texttools/merge/ModelElementChangeRightTargetMerger.java

....texttools/old/org/eventb/texttools/merge/ModelMerge.java

...entb/texttools/merge/ReferenceChangeLeftTargetMerger.java

...ntb/texttools/merge/ReferenceChangeRightTargetMerger.java

...rg/eventb/texttools/merge/ReferenceOrderChangeMerger.java