PARQUET-1323: Fix compiler warnings on clang-6

The `schema_descr_` field was unused

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #478 from wesm/clang-6-warnings and squashes the following commits:

7cadfab [Wes McKinney] Fix compiler warnings on clang-6
diff --git a/src/parquet/arrow/reader.cc b/src/parquet/arrow/reader.cc
index be270a8..0f671b7 100644
--- a/src/parquet/arrow/reader.cc
+++ b/src/parquet/arrow/reader.cc
@@ -165,7 +165,7 @@
         file_reader_(reader),
         next_row_group_(0) {}
 
-  ~RowGroupRecordBatchReader() {}
+  ~RowGroupRecordBatchReader() override {}
 
   std::shared_ptr<::arrow::Schema> schema() const override { return schema_; }
 
diff --git a/src/parquet/metadata.cc b/src/parquet/metadata.cc
index ee91b39..49999a4 100644
--- a/src/parquet/metadata.cc
+++ b/src/parquet/metadata.cc
@@ -66,7 +66,7 @@
 
 std::shared_ptr<RowGroupStatistics> MakeColumnStats(
     const format::ColumnMetaData& meta_data, const ColumnDescriptor* descr) {
-  switch (meta_data.type) {
+  switch (static_cast<Type::type>(meta_data.type)) {
     case Type::BOOLEAN:
       return MakeTypedColumnStats<BooleanType>(meta_data, descr);
     case Type::INT32:
diff --git a/src/parquet/schema.cc b/src/parquet/schema.cc
index 5c3958e..7718781 100644
--- a/src/parquet/schema.cc
+++ b/src/parquet/schema.cc
@@ -690,8 +690,7 @@
                                    const SchemaDescriptor* schema_descr)
     : node_(node),
       max_definition_level_(max_definition_level),
-      max_repetition_level_(max_repetition_level),
-      schema_descr_(schema_descr) {
+      max_repetition_level_(max_repetition_level) {
   if (!node_->is_primitive()) {
     throw ParquetException("Must be a primitive type");
   }
diff --git a/src/parquet/schema.h b/src/parquet/schema.h
index b778e51..1a94ed1 100644
--- a/src/parquet/schema.h
+++ b/src/parquet/schema.h
@@ -369,11 +369,6 @@
 
   int16_t max_definition_level_;
   int16_t max_repetition_level_;
-
-  // When this descriptor is part of a real schema (and not being used for
-  // testing purposes), maintain a link back to the parent SchemaDescriptor to
-  // enable reverse graph traversals
-  const SchemaDescriptor* schema_descr_;
 };
 
 // Container for the converted Parquet schema with a computed information from