refactor: remove state cleanup in the diff parser

This is not necessary because the state is (and must be) only used for
parsing one variation diff. Furthermore, if this would be needed, it
would be buggy. In particular, this cleanup code is not executed in case
an exception is thrown.

Author: ibbem

src/main/java/org/variantsync/diffdetective/variation/diff/parse/VariationDiffParser.java

@@ -288,11 +288,6 @@ private VariationDiff<DiffLinesLabel> parse(
             );
         }
 
-        // Cleanup state
-        beforeStack.clear();
-        afterStack.clear();
-        lastArtifact = null;
-
         return new VariationDiff<>(root);
     }