Removed yii\db\Schema from use section

Declaration of yii\db\Schema is not needed by default. Migration can be related with changing data or renaming column, so user should explicitly add it when it's needed.

framework/views/migration.php

@@ -8,11 +8,11 @@
 echo "<?php\n";
 ?>
 
-use yii\db\Schema;
 use yii\db\Migration;
 
 class <?= $className ?> extends Migration
 {
+
     public function up()
     {