Remove unused entity_id foreign key

This removes the entity_id foreign key from the
catalog_product_entity_media_gallery table. The entity_id column is
dropped in UpgradeSchema.php, so the foreign key can never be created.
This patch fixes a fatal error on AWS Aurora.

Author: mattjbarlow

app/code/Magento/Catalog/Setup/InstallSchema.php

@@ -2022,18 +2022,6 @@ public function install(SchemaSetupInterface $setup, ModuleContextInterface $con
                 'attribute_id',
                 \Magento\Framework\DB\Ddl\Table::ACTION_CASCADE
             )
-            ->addForeignKey(
-                $installer->getFkName(
-                    'catalog_product_entity_media_gallery',
-                    'entity_id',
-                    'catalog_product_entity',
-                    'entity_id'
-                ),
-                'entity_id',
-                $installer->getTable('catalog_product_entity'),
-                'entity_id',
-                \Magento\Framework\DB\Ddl\Table::ACTION_CASCADE
-            )
             ->setComment(
                 'Catalog Product Media Gallery Attribute Backend Table'
             );