Change 'Boolean' to 'boolean' in RemoveUnusedClassProperties.

The variable in question can never be null, so boxing is completely unnecessary.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177275858

src/com/google/javascript/jscomp/RemoveUnusedClassProperties.java

@@ -86,7 +86,7 @@ private void removeUnused() {
            * In some cases the parent of GETPROP is an EXPRRESULT, while the replacement is always
            * just a plain expression.
            */
-          Boolean replaceParent = true;
+          boolean replaceParent = true;
           if (NodeUtil.isLhsByDestructuring(n)) {
             if (parent.isStringKey()) {
               // Remove the entire object-key entry