perf(database): remove unused index on folders

[tericcabrel]

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[github-actions]

Deploy request opened: https://app.planetscale.com/teco/core-db/deploy-requests/10

When adding to the schema, the Deploy Request must be run before the code is deployed.
Please ensure your schema changes are compatible with the application code currently running in production.

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 CREATE TABLE `folders` (
 	`id` varchar(50) NOT NULL,
 	`user_id` varchar(50) NOT NULL,
 	`parent_id` varchar(191),
 	`name` varchar(255) NOT NULL,
 	`created_at` datetime(3) NOT NULL DEFAULT current_timestamp(3),
 	`updated_at` datetime(3) NOT NULL,
 	`path` text,
 	`category` enum('visible', 'favorite', 'archived', 'hidden') NOT NULL DEFAULT 'visible',
 	PRIMARY KEY (`id`),
 	UNIQUE KEY `folders_user_id_parent_id_name_key` (`user_id`, `parent_id`, `name`),
 	KEY `folders_name_idx` (`name`),
-	KEY `folders_user_id_idx` (`user_id`),
 	KEY `folders_parent_id_idx` (`parent_id`),
 	KEY `folders_category_idx` (`category`)
 ) ENGINE InnoDB,
   CHARSET utf8mb4,
   COLLATE utf8mb4_unicode_ci;