[tree generator] Remove Suppress("unused") from implementation classes

Those only implement base classes, their members are not supposed
to be referenced directly (those sometimes they are).
So unused code in there can be suspicious.

On the other hand, some generated builders are truly unused, so leave
the suppression for them.

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/printer/common.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2024 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -123,7 +123,7 @@ fun <Element, Implementation, ElementField, ImplementationField> generateTree(
             treeGeneratorReadme,
             implementation.packageName,
             implementation.typeName,
-            fileSuppressions = listOf("DuplicatedCode", "unused"),
+            fileSuppressions = listOf("DuplicatedCode"),
         ) {
             createImplementationPrinter(this).printImplementation(implementation)
         }