Revert "Subscription manager"

This reverts commit a612500b0f.

migration/subscription.12.cr

@@ -1,31 +0,0 @@
-class CreateSubscription < MG::Base
-  def up : String
-    # We allow multiple subscriptions for the same manga.
-    # This can be useful for example when you want to download from multiple
-    #   groups.
-    <<-SQL
-    CREATE TABLE subscription (
-      id INTEGER PRIMARY KEY,
-      manga_id INTEGER NOT NULL,
-      language TEXT,
-      group_id INTEGER,
-      min_volume INTEGER,
-      max_volume INTEGER,
-      min_chapter INTEGER,
-      max_chapter INTEGER,
-      last_checked INTEGER NOT NULL,
-      created_at INTEGER NOT NULL,
-      username TEXT NOT NULL,
-      FOREIGN KEY (username) REFERENCES users (username)
-        ON UPDATE CASCADE
-        ON DELETE CASCADE
-    );
-    SQL
-  end
-
-  def down : String
-    <<-SQL
-    DROP TABLE subscription;
-    SQL
-  end
-end