Remember extraneous Painless code (#126057)

This removes some leftover remnants from using StringBuilder 
as part of String concatenation. Since we no longer support JDK 8, 
this code can be safely removed.

Author: jdconrad

modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultUserTreeToIRTreePhase.java

@@ -15,7 +15,6 @@
 import org.elasticsearch.painless.Location;
 import org.elasticsearch.painless.MethodWriter;
 import org.elasticsearch.painless.Operation;
-import org.elasticsearch.painless.WriterConstants;
 import org.elasticsearch.painless.api.ValueIterator;
 import org.elasticsearch.painless.ir.BinaryImplNode;
 import org.elasticsearch.painless.ir.BinaryMathNode;
@@ -913,10 +912,6 @@ public void visitAssignment(EAssignment userAssignmentNode, ScriptScope scriptSc
                 stringConcatenationNode.attachDecoration(new IRDExpressionType(String.class));
                 irCompoundNode = stringConcatenationNode;
 
-                // must handle the StringBuilder case for java version <= 8
-                if (irLoadNode instanceof BinaryImplNode bin && WriterConstants.STRING_CONCAT_BOOTSTRAP_HANDLE == null) {
-                    bin.getLeftNode().attachDecoration(new IRDDepth(1));
-                }
                 // handles when the operation is mathematical
             } else {
                 BinaryMathNode irBinaryMathNode = new BinaryMathNode(irStoreNode.getLocation());