Remove usage of OPERATOR_UNSPECIFIED and use the real OR operator. (#3888)

We had used OPERATOR_UNSPECIFIED in place of the OR enum because the proto was
not ready. Now that the proto has been updated, we can remove those.

Also found an out-of-date TODO and removed it.

Author: ehsannas

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java

@@ -701,8 +701,7 @@ StructuredQuery.CompositeFilter.Operator encodeCompositeFilterOperator(
       case AND:
         return StructuredQuery.CompositeFilter.Operator.AND;
       case OR:
-        // TODO(orquery): Use OPERATOR_OR once it's available.
-        return StructuredQuery.CompositeFilter.Operator.OPERATOR_UNSPECIFIED;
+        return StructuredQuery.CompositeFilter.Operator.OR;
       default:
         throw fail("Unrecognized composite filter type.");
     }
@@ -713,8 +712,7 @@ com.google.firebase.firestore.core.CompositeFilter.Operator decodeCompositeFilte
     switch (op) {
       case AND:
         return com.google.firebase.firestore.core.CompositeFilter.Operator.AND;
-        // TODO(orquery): Use OPERATOR_OR once it's available.
-      case OPERATOR_UNSPECIFIED:
+      case OR:
         return com.google.firebase.firestore.core.CompositeFilter.Operator.OR;
       default:
         throw fail("Only AND and OR composite filter types are supported.");

firebase-firestore/src/main/java/com/google/firebase/firestore/util/LogicUtils.java

@@ -298,9 +298,6 @@ protected static Filter computeDistributedNormalForm(Filter filter) {
    * @return the terms in the DNF transform.
    */
   public static List<Filter> getDnfTerms(CompositeFilter filter) {
-    // TODO(orquery): write the DNF transform algorithm here.
-    // For now, assume all inputs are of the form AND(A, B, ...). Therefore the resulting DNF form
-    // is the same as the input.
     if (filter.getFilters().isEmpty()) {
       return Collections.emptyList();
     }

firebase-firestore/src/test/java/com/google/firebase/firestore/remote/RemoteSerializerTest.java

@@ -702,8 +702,7 @@ public void testEncodesCompositeFiltersOnDeeperCollections() {
                 Filter.newBuilder()
                     .setCompositeFilter(
                         StructuredQuery.CompositeFilter.newBuilder()
-                            // TODO(orquery): Replace with Operator.OR once it's available.
-                            .setOp(CompositeFilter.Operator.OPERATOR_UNSPECIFIED)
+                            .setOp(CompositeFilter.Operator.OR)
                             .addFilters(
                                 Filter.newBuilder()
                                     .setFieldFilter(