8335182: Consolidate and streamline String concat code shapes

Reviewed-by: liach, jvernee

src/java.base/share/classes/java/lang/System.java

@@ -2611,10 +2611,6 @@ public final class System {
                 return StringConcatHelper.lookupStatic(name, methodType);
             }
 
-            public long stringConcatHelperPrepend(long indexCoder, byte[] buf, String value) {
-                return StringConcatHelper.prepend(indexCoder, buf, value);
-            }
-
             public long stringConcatInitialCoder() {
                 return StringConcatHelper.initialCoder();
             }