Do not give a SourceSection to the SplatCastNode of a MultipleAssignmentNode

* Otherwise it is consired instrumentable, but instrumentation cannot
  replace it safely in MultipleAssignmentNode, as RubyNodeWrapper does
  not have `#execute(Object value)`.

Author: eregon

src/main/java/org/truffleruby/parser/BodyTranslator.java

@@ -2212,7 +2212,6 @@ public RubyNode visitMultipleAsgnNode(MultipleAsgnParseNode node) {
                         : SplatCastNode.NilBehavior.ARRAY_WITH_NIL,
                 true,
                 null);
-        splatCastNode.unsafeSetSourceSection(sourceSection);
 
         final MultipleAssignmentNode ret = new MultipleAssignmentNode(
                 preNodes,